According to the claimed 93db/watt efficiency rating of the Sonata speakers, the peak SPL at max power with an 8 watt amp would be 102db at 1 meter. Most amps have some degree of headroom, so you would likely be able to get maybe 104db peaks. I think this is plenty loud, but some people may differ. A good quality 300B SET should do fine with those speakers, in my opinion.
I think the one of the main reasons for using tubes is to eliminate the grain of solid state. If you just go part-way into tubes, you still have the grain of whatever SS unit you have in there. Some are better than others, but true tube-freaks swear they can hear the presence of one transistor in the circuit.
If you try a tube amp or pre, and you like what you get, then go tube all the way with your next component. I was a SS guy most of my life until I decided to try tubes. Now you couldn't give me a SS amp or preamp. I think they are better in nearly every way, in most applications. The beauty of high efficiency speakers is the ability to use a low power amp with lots of detail and delicacy and transparency. You should maximize that by using an amp that can give those qualities.
I'd recommend a used David Berning "Siegfried" 300B or 811 SET ZOTL power amp, and then get a good tube preamp ASAP. I use a Berning SET ZOTL and I guarantee you it is a totally killer amp. It is the only amp in the world that can provide true SET topology with OTL configuration. The best of both worlds. Jaw-dropping would be a good description. The only caveat is that it is extremely revealing, and will quickly let you know where the other problems in your system lie. However, I consider this a strength and not a weakness. You just have to have a front end that can stand the scrutiny of a completely revealing system.
