Replacing my Krell FPB 600 amp with a tube amp

I'd think about some Carver mono's and an awesome subwoofer (why put all the burden on a single amp?). Also, Carver claims to run his tubes in a manner that they last 20 years (instead of the usual two).

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.

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.

@tattooedtrackman "why would i need a subwoofer? "
You may not need one. You could try the ARC 250s first and add a subwoofer as needed. I got the impression that you are a tattooedBASSman and tubes just aren't going to give you the "power and bass slam of the Krell FPB 600". I'm driving my B&W 804 D3s (two 6 inch woofers each) with 300 WPC and I'm using a 12 inch powered subwoofer as well.
With cars it's cubic inches. With Bass it's driver size.

@tattooedtrackman - It appears that your ARC Ref 6 preamp has two variable outputs and one fixed line out for use with a tape recorder.
You pay your $14,000 but you get what you pay for!

@tattooedtrackman - I believe your ARC is AWSOME! I've had many ARC components over the years and I put them on a pedestal. Can honestly say that my current system meets ALL of my needs. The only way I'd enjoy it more is A. A bigger room (e.g. 25 X 50 feet)  and/or B. Legalized THC. Long live music!