Two McIntosh 275's or Single MC452???

Given the option between tube & SS amps, tubes win, hands down, every time... assuming speakers are efficient enough to be driven by a tube amp.

Agree with Pdreher. MC 275 is ballsy amp. Don't let WPC fool you.

As a long time owner of a MC402 and one who craves a MC452, as I understand it has improved the sound of the 402. I have to propose a question that only you can answer. How do you like the sound of the MC275 in you system now? If you are happy and enjoy it then the answer is simple get the MC275 and bi-amping can only enhance the areas of sound that you now enjoy, so if you thirst for more of the same add the MC275. But if you feel that you are lacking in some areas but want to try a new approach then Solid State may be the answer. My experience in all the SS MC amps I have owned vs the MC tube amps I have heard in comparison to SS amps is that (in my opinion) the SS deliver a slight acceleration in delivery, expecially in the mids. I think the tubes deliver a more relaxed bass than the SS do. But the upper-mid and highs are much more natural, and pleasant. Again that has been my experience, you and others may have a different result.

The only way to find out which works best for you is to try both.  I used the MC275 for a long time and loved it.  I now use MA7000.  I gave up a little heart and soul/midrange magic of tubes for SS bass authority and large room filling dynamic capability.  Either way, you cannot go wrong IMO.  The size of your room and choice of speakers should play a big role in the decision I think.