Those speakers are extremely efficient at 99dB. No, they don't NEED 100w...it's what they can handle continually. You could easily power them with the 15w tube amps you're looking at, and both amp and speakers are rated 8 ohms. You're good there. If you do go that route, you can buy almost any preamp you want for that system since the input impedance for those amps are so high. Check out some reviews, do some searches of what preamps people are enjoying with low watt tube amps.

Personally, I would stick with a solid state preamp since you already have the "tube" sound in the mono blocks. Whatever you choose will have a CD (or AUX) input for your CD player. It will also most likely have a Phono input for your turntable. This phono input will be MM (for moving magnet). Depending on the preamp, it might also offer a switch on the phono input to choose MC (moving coil). So if your turntable has an MC cartridge on it, you need to consider this. If for some reason the preamp doesn't have a Phono input THEN you need to look at buying an external phono stage.

And, yes, an integrated amp can be used as a preamp - as long as it has a "preamp out" option on the back. This is actually not a bad choice either, as you'll then have a backup amp if a tube goes out or something goes wrong with the mono blocks, or if you just want to experience a different sound.

The short answer is tube amps and solid state amps rated at the same power level will play equally loud, but the older tube amps had more “dynamic headroom” than solid state amps so theoretically they could play louder on peaks, without distortion. I don’t know if that’s still true, that tube amps have 3dB or whatever more dynamic headroom.

But the real issue is the sensitivity of the speakers not their power rating. High efficiency speakers can be easily driven by low power amps. That’s why you sometimes see horn speakers 102dB driven by 5 watt “mini amps.” But speakers with low sensitivity e.g., 87 dB require higher power amps. If your speakers are over 95 dB sensitivity you should be OK with 15 watt amp. But as they say the proof of the pudding is in the eating.

I think the goal for a lot of audiophiles is to have the cleanest audio path possible with the fewest parts in the way. The number of watts is just a bi-product of this choice. Look at a SET (Single Ended Triode) design, coveted by many...one device, single tube, cleanest signal, Class A...by design they're only 2-3 watts. Pair that amp with high-efficient speakers and you're golden. At least some think so. Regarding watts per channel, the mono block would be sending 15 watts to a single speaker...stereo amp rated 30 watts would send 30 watts down each channel to both speakers...not the same.

I wouldn't get hung up on number of watts anyway. More watts doesn't necessarily mean a huge jump in volume....doubling watts, say 25 watts to 50, doesn't mean it will be twice as loud. It's not a linear relationship. Doubling watts only gets you another 3dB in acoustic power...not much. If the speakers are really efficient (like yours) you can have fun with low watt amps.

Repeat, power of amps should be matched to efficiency of speakers. That’s why people use mini watt tube amps circa 3-5 watts with hi efficiency horn speakers with excellent results. You can same loudness with mini watt tube amps and high efficiency horn speakers as with low efficiency speakers and high power amps. I’m generalizing, since there are a few low efficiency speakers like Rogers that cannot take much power.

It might be OK for solid state amps but tubes are susceptible to microphonics. Better not.

Hi late to the conversation but I also have Forte IV speakers and running a First Watt F7 amp which would lean toward the tube sound vs more clinical sound. I just added the Esoteric N-05dx and have been blown away. Sound stage increased two feet and got holographic. Yes I bought the unit and it is amazing. I only stream so if you have to stay on the dark side and go vinyal I think there is a unit with phono amp. I could not be happier with the Esoteric which is preamp/DAC/Streamer.
jh