Which preamp? Opinions? Suggestions?

@Czarivey, D=duck.

why do you need class A driving bass columns?
class D can do much better job.

The traditional favorites driving IRS-Beta's were big Threshold or Krell Class A amps driving the bass columns, with ARC tubes driving the midrange/tweeter panels. In that setup, I would go for an ARC tube preamp. In your situation, for synergy I would go for either a VTL TL7.5 Series 3 (if you can afford it) or TL6.5 Series II. Apparently the TL7.5 sounds more like a pure tube linestage, but with the desirable qualities of a good hybrid linestage such as low noise, linearity etc.

C421C, from what I've Googled the Phase Linear 400 has good performance to be used as a bass amp. Wattage, current, and damping factor.
Sure there are better today, but what you've got is ok for bass, mids and highs are most probably rubbish.
Just use it as I've outlined above and all your up for is the $49 for the Schiit passive preamp, I'm sure you'll be happy.
Then later on get a better amp for the bass, maybe a class d, as that's all their good for as well.

Cheers George

with the Classe CP800, you have the ability to eliminate the crossover and get a shorter signal path. I own one and it's great! Joe

Yes, the system needs to be bi-amped.

I don't think the Infinity system allows a single amp to run full range. Isn't there a servo feedback system between the woofer cabinet and the amp driving it through the crossover?

That system may have been eliminated by that time, but at any rate I'm pretty sure this system needs a separate woofer amp.

agree with Don_c55 100%.

Drive the IRS full range with the VTL, it has enough power.

Dump the Phase Linear, it was crap in the 70's.

I would opt for a 10kohm passive volume control, as you already have the servo controller after it which will drive the Phase Linear (39kohm) from it's buffered ouputs and the VTL's (100kohm) from it's bypass outputs nicely.

So all the passive and source would have to do is drive the input of the servo controller and VTL, which it will do nicely, so long as your source has low output impedance which most all sources are.

Here is a very nice priced 10kohm passive, with source switching for $49usd, which will be almost invisable and be more transparent/dynamic than any active preamp in your situation.
Unless you wish to colour the sound of your source. Then an active preamp is the way to go as they all have differing colourations that is why they all sound different.

http://schiit.com/products/sys-passive-preamp

PS: Hookup
Source/s into the Schiit input/s, Schiit outputs to the input of the servo, bypass of the servo to the VTL's, buffered output of the servo to the Phase Linear. Set the LF level of the controler to sound is balanced with the VTL's gain. And your master volume control becomes the Schiit (shame about that name)

Cheers George

Speakers and subs connect through the infinity servo controller.

How do you connect the speakers and subs? I assume you need some type of crossover.