What Power Amplifier Should I Buy?

Two things come to mind.  The vintage pre-amp may not be up to the task. Ensure that you choose a preamplifier with 2 outputs.  Ultimately consider bi-amping, running the Bryston's on the low-end and something else on the tweeters.  Could be another stereo amp or monoblocs?  You have options with the Bryston's like "jimmy2615" stated.

@jimmy2615   @perazzi28 

I've talked to Bryston about bi-amping. They recomment against doing it because the speakers drop below 4 ohms at the xover to the highs (2.2 KHz), although they hedged saying I moght be OK because there's not much power/current needed at that frequency.

I'm not taking that chance.

Biamping will give me more power only if I use two outputs from the preamp. Not very many preamps have two (balanced) outputs. The ML 38s doesn't. But the Bryston BP-17 cubed does!

What's the word on this preamp? I've looked at it before thinking I would try it or the Benchmark LA-4. I wound up getting a Cary tube power amp. That didn't change the sound very much, so I went back to the Bryston amp.

@scm I deliberately crossover at 100-120Hz because of a nasty room resonance at 40Hz. This creates a big dip at 80Hz that can't be EQed away very effectively. This dip is very strong with the f208 speakers. It is much smaller with the sub. I've placed the XO frequency a reasonable distance from that resonance. I've used REW and other tools to guide me.

I've listened to sub only at 75, 100, and 120 Hz. I cannot hear voices at all. Frequencies below 200 Hz are radiated into a half sphere, with no dependence on angle. How can this muddy the sound?

Of course, I could be wrong. Could you explain how this higher XO frequency creates muddiness? Thanks.

Two subs may be the way to go.

@kevemaher thats why I recommend not using them as monoblocks, which is what Bryston is talking about. In this case they are bridged and each amp becomes one channel and doubles the power into 8 ohms. Try to find specs though on what they put into 4 ohms when bridged. It’s not recommended for speakers like Revels.
If you use one channel though from each amp to power the highs and lows of a speaker you let the amplifier ‘see’ a much easier load than if it was powering the whole speaker, and offering much more headroom. Active speakers (like ATC) have an amplifier for each driver in the speaker. You’re right, and as I mentioned in my first post you need a preamp with two sets of outputs or you can get a y-splitter for your preamp. Incidentally, when I had two 2.5s I tried them in this configuration as well as bridged monoblocks, powering the Revel 228 Be. It sounded ok as monos but not as good as the other way. A bit muddy. And two amps definitely sounded better than one, but the difference was not huge.

I don’t think you could go wrong with a 3B or a 4B, but I was looking at it from a price perspective. Probably cheaper to pick up a used 2.5. But also, from a power perspective, 2 smaller amps may be better than one larger. The 2.5 is conservatively spec’d. It’s basically a 150/300 w amp into 8/4 ohms. So biamping, at 4 ohms you’re getting 300 w/ch into the mid and tweeter, and 300 into the woofers, and each amp channel sees an easier impedance load. With something like a 4B you would get 500 w/ch into each speaker and it will see the combined load of the whole F208.

The BP17 is great with the 2.5, best match I had with those amps. There is a brand synergy there for sure. I haven’t heard the BP19 or BR20. The Benchmark LA4 is one of the best preamps I’ve ever had, especially as far as neutrality goes. It is a gem and at its price a tremendous deal. But for synergy with Bryston amps I’d probably get a Bryston preamp, if it were me. Plus as you mentioned you get the dual outputs.

Lastly, another option if you want to stick with Bryston and/or save some $$ and shelf space, the B135 integrated has preamp outs.  So you could use that with your 2.5B to horizontally biamp.