There's a sound I get out of my SVT that I just can't seem to duplicate with any other amp. I can get real close with the Pro 3 and the Pro 4. Never played through a Pro 2 so I can't speak for that one. I've heard a Tonehammer 500 that also seemed quite close. All of these were played through an 8 10e cab. Will the audience notice the difference? Probably not......but I do so I'm still using the SVT. As I get more geezerly , will I ever switch to a PF or a Tonehammer? Maybe........but not quite yet.
As to trying to describe the difference between them , not even gonna try. You have to hear them to really get it. I will go as far as to say , the SVT just sounds fatter to me.
As for other SS amps , I haven't heard any that come close to an SVT. Maybe they are out there , I just haven't heard one.YMMV
edit: I did hear an old Peavey head through a Genz Benz 4 10 that sounded darn good a couple nights ago ,but TBH, I find my 8 10 easier to move around than a 4 10.