Ideas for low cost amplifiers with gain control

Well that's an interesting application. Parasound Halo A-series are good amps with input sensitivity / gain pots that would work well for your needs. Not sure if you have criteria for new versus used. Two used A23 would fit well within your budget. Or a used A21 (better amp) for the main pair and used A23 for the off-axis. Halos would be off the table if you want to buy only new. 

Fosi V3 for $100. They have adjustable gain. Buy two. Why pay more? 

If this is just for a proof of concept:

1. Two cheap integrated amps. You can sit in the chair and tap volume controls to test

2. Get some older 5 channel hunk of iron like outlaw or emotiva and put a minidsp in front of it to control gain via laptop in the chair

NAD C268 or used older models.  

Helpful responses. Thank you. The matching preamp to the Parasound A23 has high pass and low pass filters, providing even more control.

An old honkin 50 lb AVR from craigslist should facilitate your proof of concept for less than $100.

I second the used 50 lb AVR idea. A friend has an older upper-lineYamaha AVR from the Adventage series with the bi-amp option where he drives big Martin Logan electrostatic loudspeakers (Quests), a demanding load. And it sounds really good!

Dayton Audio APA150. Very good amp for the money, has gain, can be switched to mono mode, and can be daisy-chained. 

I use an older Macintosh MC752 to drive the upper end (and a MC300 for the lower) of my Klipish CF 4. The adjustable gain works well to tame the horns along with the McIntosh “warm sound”. This amp combo brings out the best of these speakers!

Downside is it’s not an easy amp to find and it’s a little pricey. But I love my little rugged 752.

I ran two pairs of Martin Logan speakers (Motions) for years, each pair powered by individual NAD amplifiers - the NAD C298 and the NAD 275BEE when I last had my basement bar room setup with multiple speakers (for two channel listening). 

Before I purchased the C298, I used a Parasound 2125v2 (about $900 new). The NAD 275BEE is readily available in the used market for around $800. 

Many Parasound amps have adjustable gain for both channels, which is a nice feature if your setup is "off-axis awkward". NAD generally provides one gain setting for both right and left.

My take on Parasound vs. NAD is that Parasound is toned for TV surround sound/dialogue and somewhat "dry", while NAD is more musical. After a couple of hours the NAD 275BEE got warm; the newer Parasound never got warm.

Both the 275BEE and 2125v2 have the ability to utilize 4 speakers each, so you could buy one and run all 4 speakers on it to see what issues you would need to work out with respects to getting the best sound from your listening position(s). There is something to be said about filling a room with music and sweet spots may be numerous and vary throughout the room.

If you have a room that carries some size and can fiddle with room treatments (no wife factor), you may not need to use gain settings at all, after you use other methods to deaden sound in one area, or introduce more sound to another.

My point being that if you can get away with not using gain, you are likely better off, but all that depends on the symmetry- or lack thereof - of your room. While gain can be a useful tool, most manufacturers will suggest selecting "fixed", which in most cases will provide around 29 db of fixed gain. Gain does match sound and it is important to ensure one speaker- or pair- does not overwhelm the other pair, but achieving what you want organically may be possible and the better solution.