I heard an interesting transformation of B&W 801 Matrix S3 speakers driven by Pass Aleph 1.2 mono blocks (200W/Ch). They became smooth and rich even in the treble, which before had always had a bit of harshness to them. So budget permitting if it were me, I would consider a Pass XA amp to replace the Krells. Rowland amps are also "tube-like" although rare on the used market. I am also not sure that your speakers really need 600W to begin with. One caveat- I agree with Mr. Holter that the Krell FPB's are very good amps- I had one myself and it was a joy to use- reliable, quiet, unflappable, easy on the eyes, etc.
Replacing my Krell FPB 600 amp with a tube amp
Im thinking of replacing my Krell FPB 600 amp with tube amp . I really love the power and bass slam of the Krell FPB 600.Could i possibly find that with a tube amp? I was thinking of the ARC Ref 250 mono blocks. I am also using an ARC Ref 6 preamp. The only thing i can think of with this much power and bass slam would be the ARC Ref 750se. Any other suggestions and how would the ARC Ref 250s as compared to the Krell FPB 600.