Replacing my Krell FPB 600 amp with a tube amp

Probably the ARC Ref 150? Just make sure your amps have equal gain and it really helps if you pre-amp has a variable "pre-amp out" as well as a "main out". Otherwise, you'll have to use a splitter to feed both amps.

A point to keep in mind regarding passively biamping the FPB600 with a tube amp, in addition to the gain matching concern that was mentioned, and in addition to the possible effects on coherence of driving the speaker with two very different amps, relates to the fact that in a passive biamp configuration (i.e., without an electronic crossover "ahead" of the amps) both amps must put out voltages corresponding to the full frequency range of the signal.

Your speakers have impedances in the vicinity of 4 ohms in much of the bass region. The FPB600 is rated to provide up to 1200 watts into 4 ohms. If you were to passively biamp it with say the ARC Ref 150 you would be reducing the usable power capability of the FPB600 from 1200 watts to not much more than 150 watts, because if you were to turn the volume control up high enough to utilize more of its power capability the lower powered amp would be driven into clipping.

Good luck. Regards,
-- Al

I don’t see any advantage to bi-amping in this case. That’s why my first suggestion was for tube amp(s) with a subwoofer. I, myself, am using a BAT integrated (mid & treble) and using the "pre-out" to drive a Bryston 4B3 (bass). The BAT is 150 WPC @ 8 ohms and the 4B3 is 300 WPC (twice as powerful). Since the gain of the amps is the same, 29db, the volume is the same. There is just more headroom for bass dynamics.
The crossover issue sounds like it is handled by the internal crossover of the speaker. The mid-treble speaker input just ignores any bass content and the bass speaker input ignores the higher freqs.

dweller.....My Arc Ref 6 does not have a variable preamp out...                       almarg.....Wow i would of never guessed that. Thanks...                                 dweller..... I too now dont see any advantage to bi amping                               dweller......Question....If i was going to go with tube amps , say ARC Ref 250 se, why would i need a subwoofer? My B&W Matrix 800s have plenty of bass, even more so with my Krell B&W Bass alignment filters when is called for.

You could try a higher-powered VTL amp, but I still don't think a tube amp will control your speakers as well as the Krell does.