Step Up from a Sunfire

What amps would be a significant improvement over a Sunfire Cinema Grand?

It is currently bi-amping a set of Maggies
Agree, send to flannerys! I bought a b stock sig 600 first gen, sent to flannery, and he said there was no blackening what so ever anywhere, so mine was either not used much, or lightly used, was happy to hear that!  He did his restore of caps, removed all the pacom  caps and replaced with better , new light, checked everything out, clean bill of health, I should be good for 15-20 years now. Worth the. Money, as. Mentioned, the power does seem endless, no matter volume, or type of music, no clipping, whining, just gobs of uber clean power whenever it's called upon. 

They are Carvers top amps, all should be judged by it.  
Unless you WANT AND NEED NEW, look at the Sanders sound system MAGTECH. VERY SIMILAR LAYOUT. Very uncanny. Similar designs. 

Stick with the sunfire fire until the humming starts, then its trade up time.  Once she visits flannerys shop for her internal facelift, you will be good for another 20 years, as long as the hum don't start. 
Mine is amazing! 
I only plug mine the morning I know I will use it that night or for the weekend.   Otherwise it's unplugged.  Lightning storms, electrical current bursts, brown outs, etc not good.
 It's fine, don't worry.  Plug in when you want to use it, but there is a good 2 hour warm up I have noticed, it will sound tighter and just more at ease when warm. 
I leave my 300 turned on 24/7 (with surge protection). It does take a couple hours to reach it’s optimal performance after a cold start. Bob Carver has written that it can be left turned on and that constant voltage to the unit will not hurt. (not a good idea if your AC is unstable, IMO).
Plus, I don’t like the idea of an in-rush of current every day when it’s being powered up.

I too have had my amp refurbished by Bill Flannery. I have a 300 first generation, and he upgraded it to meet the specs of the Sunfire 300-two (3rd generation). It now has a much lower noise-floor.

In my situation the sunfire sound is effortless . From low , med to high volume it does not change it’s presentation . No nasties in any frequencies . Smooth sounding .
@maplegrovemusic, that says it all. I can only add that the Sunfire "house sound" is lush, warm, with wide and deep soundstaging. It’s very "non-solid state" sounding.