Passive pre-amp any good???

My experience w/ passive preamps is limited. I have heard the ASL $500 xformer-based passive pre. It was driving an ASL power amp. Maybe matching was not an issue as both were from the same company?
The sound was exemplary, if I may say so! No thin sound at all.
The sound was very transparent & since I was listening to single-driver speakers (Cain & Cain Single BEN & Carolina Audio JTM) the sound was so very coherent!
I have a hard time understanding why an amp input would need a current drive! In my books, such an amp would a failure of a design! A power amp is essentially a voltage source so it's voltage in & voltage out (with high current drive). The input to a power amp is usually to MOSFET gates or to a BJT base both of which are high impedance & small signal. Current into a BJT base should be in the 1uA or less, if the beta is high (today this is *practically* guaranteed).
I'd believe you that you have heard a thin sound but I would not jump the gun & pin it to lack of current drive from the source component. It is *possible* that the passive pre hadn't broken in OR it might be possible that the passive finally revealed the true colours of the source component!
IMHO. FWIW.

I went from a modest powered preamp (Forte model 2) to using a separate phono preamp unit (benz pp1) with a passive pre unit and the difference was night and day in terms of expanded, not reduced dynamics, better and lower bass information, wider deeper soundstage, cleaner more revealed information. I would never go back to an active preamp unless it was one of those $10k plus designs (FM accoustics or something like that) it would have to rival the passive. Only thing with the passive is that you cannot get that extra very high volume (if you want to play loud rock) but that can be overcome with efficient speakers. ( I have a pair of Jbl L-200's for that purpose)

P.S., the airtight amps.. don't they offer a volume control anyway? they are supposed to be exceedingly good. First Sound is one of the best passive preamps around and is highly recommended

A good passive will not limit your dynamics or roll off the frequency extremes. Also, most sources today will drive any power amp to clipping. Gain is rarely needed (at least in the century).

Placette Audio and Bent Audio both make very transparent passives at reasonable prices. Placette even offers a full refund if you don't like the unit.