Can I use my passive preamp with tube amps?

Passive attenuators can be very transparent/faithful to your signal, regardless of amp topography. In addition to the above: Listening satisfaction will depend on the output voltage of the source/sensitivity of your amp. If the source(CDP, DAC, phono pre, etc) isn't putting out enough to drive your amp to adequate SPLs; you won't be happy. Having owned the(most excellent) Placette Passive Linestage, I found that some pressings, recorded/engineered at lower levels, were less than satisfying. Then again- I like my music loud, and most of my collection produced excellent results.

If you're asking why he'd want to go to tube monoblocks: If you really need an explanation, you wouldn't understand.

Perhaps he wants his music to sound real, rather than to sound like audio. He did say that he prefers the sound with a passive. Stan's comment is right on the money. The goal used to be called, "straight wire, with gain."