The only way to determine which component is the limiting factor is to actually do some experimenting. If you can borrow some alternatives, you can at least determine if available power is the issue. You could also try different tubes to see if that makes a difference. For example, my amps came from the manufacturer with fairly expensive 5692 driver tubes, but I found alternatives that, to me, sound better.
I like SET amps myself, but if you need more power and don't want to give up too much of "that sound," you should consider pushpull triode amps. I personally find that the kind of tube (triode vs. pentode) is more musically significant than SET vs. pushpull. Simple pushpull amps can sound very, very good. I generally don't like the sound of most high powered pentode amps -- they tend to sound brittle and dry to me.
I am not familiar with the VS DB-99, but I find a claimed "in room" 99 db efficiency rating to be obtuse. What does that really mean and how does it translate to more standardized measurements like db at one watt or 2.83 volts at one meter in an anechoic chamber? If I add in room reinforcement, plus the contribution of the other speaker in a stereo pair, which is, after all, "in the room," I could turn a 93 db/watt speaker into a claimed 99 db speaker. Who knows if even more liberties are being taken with that rating. Consider too, that speakers with the same rating can be dramatically different in how difficult they actually are for amps to drive (depends on the impedance curve and phase angle).
