Hey guys, I know I'm coming into this rather late, but I will post my opinions nonetheless:
I like a high-pass. Many would rather do it in post, but in a boomy room you may as well cut the bass so you can fill the "meat" of your recording with usable data. Why cut the top 5 or 10 db from your wav and then boost everything back up in post? It is just as easy and no less destructive to add some bass if need be, but you may as well go for the gold if you know the room. Maybe 120-150hz w/ a 12db slope.
When I think of a -20db pad I think of a pre that is not geared toward concert recording, or one with built-in adc so you can take a board-feed if need be and use the unit just for the adc. This may help you appeal to the nature recording-dictation-ribbon mic using crowd and if so, I'm not offended by it.
Gain can be in steps, but it is easier to fine tune from the pre. I'm assuming using steps will help quality and that is all good. Maybe 2 or 3 db steps in the most usable areas and 5db in the extremes. However fine tuning with your recorder makes your end result subject to the quality of that pot right? So wouldn't it make sense to have that control onboard the pre and leave the recorder at unity?
I like meters on the pre, but IMO a -3db clip light for each channel should be minimum. Unless you are monitoring you could be clipping the signal and never know it. The recorder will tell you the level but not if it is clipping before it gets there. Maybe if the pre's clip freakishly high so this is never a concern all is good, but I still vote for some blinking lights.
Flexible power is nice... 12v is hard to come by these days except bricks, and by keeping in multiples of 9v you should be rockin. I would not recommend getting into the battery business though... why deal with the warranty and liability of the power source? I think you are on track with just including a couple connectors for diy use. A power transformer for A/C use would be a nice option for those of us who would use this as part of a multi rig when we have power.
I would love to see a headphone amp also... clean is less important than loud IMO. 250mv is about right. I often will tweak my mic positioning during the first song of a show based on my monitoring.
Ergonomics: I think you would appeal to more markets if you can make this somewhat modular. That is to say, if you make it 1/3 rack size, then come up with a rack mount system so you can marry 3 together in a rack for studio/live use. Also the placement of the ins/outs should be on the back so you don't have to worry about your cables getting in the way of your controls. All controls on the front is best for bag (or rack) use. If you like the modular idea, keep the sides clean. Unless it is stealth-sized xlr in/out is best, but for a short run to an adc unbalanced is fine IMO so 1/4" outs should be cool. I would also put an 1/8" stereo jack for use without outboard adc to a small recorder that is either a patcher or main unit.
There was some discussion of 4 chan, and I assume you are set on doing this as 2. That is cool and wise. My opinion on 4 mics is mixed (pardon the pun) and I have done it certainly, but in most cases it was as a multitrack. There is one particular recording that I did as a 4 mic live mix that I really like, but it was done with a pair at center stage and a pair of omni's split to the stage corners for crowd response. It was dumb luck that this recording turned out how it did because I had troubles getting the board feed that I planned on mixing in. On top of that, my multitrack failed and it was just my 4 track live mix to my jb3 that survived. I would try it again, but as soon as you get any front to back distance between the mics you have to worry about delay and that is multitrack land as far as I'm concerned. That all being said, if you were to dive into a 4 chan pre, and wanted to do it right, you could do the first 2 channels as you are now, but make chan 3 and 4 a simpler non-transformer circuit (fast,transparent op-amps), with combo neutriks, a mono switch, and a polarity reverse switch. In addition you should have 250+mv headphone w/level, individual monitoring for each chan, -20 db pad for a line-level signal, individual meters, an overall meter, overall balance, main out level control, 4 mono outs as well as a 2chan mix, and with a little more thought I could come up with a few more things that would make this "usable" in my eyes as a small format matrix mixer. Truth is though, it is a rare occasion that you need portability for running a matrix, and running a 4 mic mix is not the holly grail. Keeping it simple may be cool for those times when you are at the venue and decide last minute to do a matrix instead of ambient. So if you do it, I would just keep it as 2 trs ins, and a stereo level control (on the front!) would be fine. A beefy headphone amp and some sort of level meters would still be important to me. Those wanting a 3/4 mic mix could use a second pre into the trs jacks.
However you slice it Chris I think it is great that you are opening your ears and your mind to the thoughts of the community. Thanks for doing that, and good luck with the project. I'm going to be very anxious to hear the comps as they start rolling out!
My .02
Matt
