The short answer to question number 3 is go with the variable outputs or read on as to what worked for me.
It might be an issue with my 1700, but it sounds cleaner through the Tape Inputs versus the Aux Inputs.
In both cases, I found my stand alone Audio-gd DAC, combined with the 1700's preamp produced too much gain, I was going to abandon my MAC 1700, when as a last try, I inserted the Japanese version of the
FX Audio (model number FX Audio 01J) in between my DAC and the Tape Inputs.
The Japanese version adds a critical switch, which enables you to toggle between zero gain and minus 6 db's of gain.
The minus 6 db's setting did the trick, I now love the hybrid sound of the MAC 1700 with the FX Audio up front.
Your tube choice for the FX Audio is very important, after rolling in a couple of dozen NOS tubes, the 6AK5's beat all other varieties, fortunately my local surplus electronics store allows you to return tubes for up to 30 days as long as you don't abuse the privilege. It takes me less than 5 minutes to A / B compare any two audio components and after trying everything, which they had, I settled on a couple pairs of RCA 6AK5's with silver (nickel) plates and large "D"getters.
Then I asked my long term vacuum tube dealer, which 6AK5 he thought was best, he sent me a pair of Ericsson 5591's with gold pins, which had nearly the identical construction of the RCA's, except for the gold pins.
The Ericsson's were a little pricey, but with only 2 tubes in my system, not counting the tubed tuner, I splurged because I love every Ericsson, which I have heard and with the minus 6 db's setting the tubes will probably out live me!