SET shortlist to drive tweeters in active system

Lewinskih01- I think what has me baffled is why you would want to use an SET on the tweeters in a 3 way. The vast majority of the musical information is conveyed by the midrange and that is where SETs really deliver their magic. Also, I believe that one of the knocks on SETs (I don't know if it's true) is that they are bandwidth limited, which again would argue against using them at either extreme in a 3 way.

Fair enough.
Maybe I was jumping to conclusions.

My 4-way would be divided as follows:
20 to 80 Hz by active subs
80 to 600 Hz by two 10" midbass woofers per side, driven by class-D amp.
600 to 2500 Hz by a midrange. Was assuming my MC275 for this. Most midranges are somewhere in the 94 to 100dB sensitivity and 4 ohms.
2.5 to 23kHz by the TPL-150H at 102dB sensitivity.

Do you think I'm better off using SET in the midrange and having the MC275 do the higher frequencies?

A number of people who choose this approach will often use a 45,2A3 or 300b. Some opt for 300b SET midrange and say a 45 SET on the tweeter. I find that high quality SETs(300b.45 or 2A3) have very natural extended high frequency sound. I agree SS is more likely to provide a hyper extended sound if that's your desire. There are certain clichés concerning SETs that don't bear out once you've heard a good example of them. You could just as easily say SS presents a tipped up upper frequency(not true for all of course) clichés are easy.

Would a SET be the very best choice for bass/subwoofer duty? No they wouldn't be. Midrange and upper frequency they'd be my first choice. Depends on what sound quality objectives you have, as is always the case. When I use the premium level 300b tubes(currently using the Elrog 300b, there are other fine choices also) the upper frequency information and air are superb and just sound natural/right. Best of success to you on your project.
Charles,

I don't see it mentioned but you will probably need some kind of external active crossover to get decent sound.

To me it just seems unnecessarily complex and violates the KISS prime directive. I'd bet you could get better sound any way by equalizing the active subs and simply using the MC275 for the rest of frequencies.

Hew,

I didn't wanted to go there to avoid complicating the thread as my setup is rather unusual. My only source is a very optimized PC running, among other things, Acourate for digital room correction. It can also perform digital crossovers, linearize drivers and time align the different channels. This will feed an 8-channel DAC that in turn will feed the amps driving the speaker drivers directly.

I realize this is not for everyone. Not sure KISS is too applicable. Passive crossovers are used to make a speaker easy to use, not for their better performance.