As someone who builds tube gear and owns many pieces from various manufacturers, a couple pieces of advice when buying tube gear.
1 Understand the tube being used. Look at the tube data sheets. Look for high transconductance, greater than 6500 ohms
2 Build quality is paramount. If AC heaters are not done right your signal to noise ratios will be poor
3 Older products may needs rebuilt. Many of the large capacitors go bad after a 20 years
4 Stick with a 9 pin tube as they provide internal shielding. Unless it has a good DC heater supply
5. Do a little research on what a SRPP, Mu Folllwer and White Cathode folllower is and what their pluses and minuses are
6 Finally the quality of parts is paramount. Poor undersized caps can lead to poor frequency response. Carbon resistors color sound
As to passive preamps a good transformer volume control costs but they are very transparent and have plenty of drive as the excess voltage is converted to current. With a tube preamp your minimum gain will be 20 which is 20 more times than your amp needs. Which means the rest of it will be burned across a carbon pot. Unless a good stepped attenuator has been used
BTW my main two amps are a parallel SET 300b and a 845. I also have a MC225, a VT100, Sophia Baby Amp, and a Unison Reaerch 845. Just to say I am not against tubes.