Hi Dan -- If I understand your question correctly, are you in essence trying to convert the loudness switch so that in one position, the unit provides the sound of the stock sonic performance, while in the other, that produced from the modifications?
If so, then a couple of thoughts come to mind:
1. From the standpoint of response characteristics, there were changes made to the Line Amp stage, Tone Amp Stage, Power Amp AF Amplifier stage, and NFB circuit to flatten the response of all of these stages/sections. Trying to use a simple DPDT switch to switch all of these modifications in and out could only be done with mini-relays, which would become quite complex.
2. The biggest contributors to flattening the response comes from bypassing the tone controls, and installing the buffer stage. The tone controls can already be maintained in the signal path by way of the Tone Control Bypass switch, and the Buffer circuit could in fact be effectively placed in and out of the signal path by repurposing the current loudness switch to provide that function.
3. Parts of the total modification like the installation of EFB and improving the phase inverter stage drive have no effect on circuit response, and you would likely want to leave these elements in place regardless of how you might proceed with any other portions of the modification.
4. After having lived with the modification now for a period of time, have you come to miss the warmer, lush sound of the original design? This is hardly meant to criticize the original performance, as so many people do enjoy it immensely. After all, it is ultimately enjoyment that we seek from this equipment, and if it doesn't provide that, then why have it?
If that is the case, then I would first consider trying to re-achieve that sound with careful use of the tone controls. The problem with so many in today's audio hobby is that they want their equipment to sound exactly like they want it to with all the controls set for flat. Then they have REALLY achieved high fidelity sound! So they go through all of these efforts to modify, modify, modify until they get what they like with the tone controls all set for flat -- except that almost always, the real response produced by such efforts is anything but flat. But, they can at least "show" that it is flat by the control setting positions, and fool themselves and others then because of their unique and highly calibrated golden ears.
The problem with this approach, is that any reference to accurate, flat performance has all but been lost. It is the same as having no point of reference to know how to proceed with a map or GPS system. Either way, you're lost. I mean no disrespect to those who play out this scenario, but it often ends in a futile search because they have no frame of reference from where to start.
On the other hand, I would much rather produce a known standard of performance with regards to accurate response with the controls set for flat performance. I may not like the tonal quality produced at that setting, but at least then I can have a solid frame of reference as to what accurate, flat performance really is. From there, I can adjust to suit, having full knowledge then of just how much my preference for reproduced sound deviates from that which an uncolored response delivers.
Others often rail against such efforts, claiming that there are a million points in the recording/delivery chain where response is/was intentionally/unintentionally altered. True enough, by why clutter it up with one more piece of equipment that further colors the response as well? How much better to know what the original signal source truly represented so you can make informed decisions from there.
In the end, designing for a flat amplification presentation has served me very well indeed over the years, and is the approach used with the modifications offered for the 400. As I mentioned in my response to you in the X-1000 thread, when various units are all designed to produce truly accurate, flat performance from the get go, then their sonic differences -- from a response standpoint -- all tend to largely disappear, so that other characteristics such as power, damping factor, features, etc., then tend to dominate the differences between the units.
This may tend to take some of the "romance" out of the equation, like how some may prefer some specific tubes, designs, or component brands over another. For those, I am hardly trying to burst any bubbles in that regard. However, there is a cold, hard truth to these facts -- that may not be all absolute in defining sound that we perceive, but none the less, play a significant factor in it. The rest is emotion. An important part to be sure, but one that should always be kept in check with the reality of measured performance -- not to make that more important than the emotional pleasure received, but to maintain the all important frame of reference.
Dave