Listening to this modified Carver M-500t MkII some more, I have a few more things to say about it...
It has a way of pulling out delicate details in the music, very light on its toes so to speak. Everything is super clean and clear. The words playful and lively comes to mind. I guess you could say it has the same performance traits of a good ESL speaker. Instant transients, uncolored sound with loads of detail and still remaining refined and non-fatiguing.
I'm listening to albums that I know like the back of my hand, and I'm hearing new details within that I never knew was there before. There's more acoustic space that can be heard, more decay in notes, shear black silence, extra textures in the instruments, and a warmish cozy feeling.
Does it sound like tubes at all? No, not really. Unless you consider the non-fatiguing sound and acoustic air along with the delicacy within the details as tube sounding. Then in that case, yes, it sounds like tubes.
And something I learned this evening about the MkII mod on these amps... Not only do they replace and upgrade every thing electrolytic capacitor in it, they also upgrade all of the diodes in the power supply as well as some kind of up-sized/upgraded transistors. And speaking of transistors, they replace the OEM Toshiba output devices with new and much larger Sanken output devices. These are the same used in a LOT of high-end audiophile amplifiers. These alone probably attribute to most of the sonic improvements in this amp.
I have to say, I'm really impressed with this amp. It has well exceeded my expectations by a long shot!