Replacing my Krell FPB 600 amp with a tube amp

What specifically is it you are looking for in a tube amp that you feel the FPB 600 fails to provide?

Are there elements in the FPB 600‘s repertoire that tube amps fail to emulate that you would find difficult to live without? Ones you may have grown used to without realising?

pesky_wabbit ....Actually i LOVE my Krell FPB 600. I actually feel like it supplies everything i could possible want in a solid state amp and more. I just thought if i am missing something with a tube amp that has that much power and some warmth that tubes provide. I just bought a brand new Ref 6 ARC preamp and besides the ARC Ref 5 se that i only had about 2 or 3 weeks before i demoed the ARC Ref 6 which i found it to be a night and day difference bet the 5 se. This was the first time i ever had a tube preamp. So with that said that is why i was thinking about a tube amp. Another words i would never go back to a solid state preamp. The only thing i could possibly see myself buying to replace the Krell FPB 600 is the ARC Ref 750 or the 750 se amp. for its power. Also my Krell FPB 600 sounds like a match match made in heaven with my B&W 800 Matrix. But always wondered what a powerful tune amp would sound like with my B&W 800s. 

A little off topic . I converse with an Audiophile that collects the Big vintage JBL’s ( 15” woofers ) . He runs McIntosh tubes and class D bi-amped with active an crossover and DSP. Not my cup of tea , but if done right , really impressive . Also no need to worry about subs . I think with B & W or ATC speakers  it would be even better . Cheers , Mike B.