There are a few things that need to be cleared up to preclude wild goose chase/false assumption.
How the Crown operates:
With the Crown input sensitivity set at 1.4 v anything coming from the preamp that is in excess of 1.4 V (input sensitivity can also be set to 0.775 v) will run the Crown at maximum level.
That's yes and not necessarily.
Yes, if the amp is running full range
Not necessarily if you are using low pass filter, which you are. I will explain this in a bit with pictures even.
The Modwright specs show that it will put out 37 Vrms max. Therefore we have way enough output per spec to run the Crown at full bore. Supporting this theory is that the Modwright has been driving other amps and speakers fine so I am not suspecting it at this point.
Again, this is yes and not necessarily.
Yes, I believe it can put out 37V if Modwright says it can. HOWEVER, output voltage is contingent on input voltage and gain of the unit. Modwright says the line level gain is 11dB. That means to get 37V out you'd need a minimum of 10V from your sources. 10V from sources is uncommon so the prospect of actually getting 37V out is unlikely in practical use.
For the Modwright to put out the 1.4V requires at least 0.4V from the source. But, 1.4V is not all as it seems, as I will get to.
I did not previously understand the led meters on the crown. This is how they work: the bottom LED indicates that there is an incoming signal but it doesn't tell the strength of that signal. The LEDs above the bottom indicate the output signal strength. They climb up to indicate increasing output power all the way to clipping. In my tests the input signal is present but the output signal meter won't light at all until the preamp is dialed up to 3/4 full and then the first output lamp just barely lights up. This seems odd to the Crown tech and to me.
The pots on the front of the Crown are attenuators. At full clockwise the crown is passing all the incoming signal through. Therefore any signal above 1.4 v should have the Crown blasting.
The Crown tech felt that the lack of output signal meters lighting up was odd given what we discussed.
What the Crown guy told you is basically true, IF the amp is being run full range. But, you are NOT running the amp full range, you're running with ~80Hz low pass filter. What this means is the output level LEDs reflect only the output content below the low pass filter. And, at 80Hz and down, the average content of the signal is typically just a small part of the total so the meters won't be expected to show much, relative to full range signal.
To put this in rough context, for sake of discussion, let's say you have the preamp turned to the point where it is putting out 1.4V signal. The presumption is this should drive the Crown to full output...and it would (or at least close) if the Crown was amplifying 100% of the signal.
But, the Crown is not amplifying 100% of the signal. It is only amplifying the portion of the signal that is around and below the low pass filter setting of ~80Hz. So, if ~80Hz and below signal only constitutes 15% of the total, then the amp is only amplifying that 15%. That is why the meters don't show what the Crown guy thought they should show or told you they should show. The following pictures illustrate this using my XLS 1000.
This first picture is baseline. The amp is set in bypass so it's running full range. I have a 1kHz signal going into channel 2 and turned up the signal generator to make sure the clipping indicator is lit up. It's about 1.6V going in.
This next picture is same signal going into the amp (1.6V @ 1kHz) but I have now engaged the Low Pass Filter, set to 79Hz. As you can see, the meter shows only signal present, but not enough to light up any of the output indicators. Which is exactly what you're seeing.
This next picture is same low pass setting of 79Hz, but now I changed the signal to 1.6V @ 250Hz. As you see, even at 250Hz, the amp shows only signal but no appreciable output, because the low pass filter is still blocking it.
This next picture is same low pass setting of 79Hz, but now I changed the signal to 1.6V @ 125Hz. At 125Hz, the amp is just beginning to let some of the signal though as we approach the low pass setting. -20dB output represents 1/100 of the amp's power, or about 2 watts.
Things start to get interesting now. LPF is still set at 79Hz, signal voltage is still 1.6V but now @ 100Hz. Just a little change in the frequency from 125 to 100Hz has the amp now 10x more output because the filter is letting more of the signal through. -10dB is 1/10 power or about 20 watts.
In this final picture, LPF is the same 79Hz, signal voltage is the same 1.6V, but now the signal frequency is 60Hz. As you can see, the amp is back to clipping level. The input signal is now below the low pass frequency so it all, or at least most of it/enough of it, gets into the amp and thus is reflected in the power output.
So, hopefully you will now conclude it is unlikely the Modwright is broken and it's unlikely the Crown is broken. The amp and meters are simply exhibiting the effect of amplifying a limited portion of the total signal because of the low pass filter.
If you want the bass to match up more with the mid/highs, I believe you are going to have to attenuate the signal to the mid/high amp or boost the signal to the Crown.
He suggested a test of the Modwright output signal using a phone for a tone generator plugged into the Modwright then testing the RCA outputs on the Modwright. The goal is to confirm what signal voltage the Modwright is actually producing. If it is producing anything above 1.4v then the crown should be working at all volume levels and should be able to be driven to clipping using the attenuator pots on the front of the Crown.
You don't really need to do all that to test the theory. Just disconnect the speakers from the Crown so you don't blast out the jams (it will be perfectly fine without a load). Set the gain controls/attenuators to max (full clockwise) then switch the amp from Low Pass to Bypass so it's running full range. Put on some tunes and turn up the Modwright. I reckon you won't have much trouble making the meters dance, especially if you put it on the 0.775 sensitivity.
I usually start with the least efficient driver, almost always the low end. Bring the others up to match. Right now you're doing it the other direction, it's like running after a speeding bus, setting the gains on the most sensitive first is like giving the bus a mile headstart. two massively different amps running two massively different drivers will virtually never have identical gain settings unless the imbalances are corrected with an eq or active crossover beforehand.
