All I can say from personal experience and a technical review on the original Micro preamp which then was a steal for £150, is that the *preamps* are technically fine, valves or not and measurement issues were from test equipment with low input impedances being used from the tape outputs! The Stereophile review on the current Integrated baeing a classic case in point. My last Series 4PP preamp was a bit of a softie on the line stages and the phono stage was load critical if the tape record outs were used, far more than the 25R that's descended from it, but Glenn helped me go half way with some beneficial tweaks and he would have happily had it back to finish off the updates, mainly on the phono stage, which on his early models was load critical (mine had 500k log pots and some had 1M I think - not needed now with his current design).
The Series 7 power amp series should be regarded more as a valve amp as it measures that way and sounds delightful into suitable speakers.
Please don't worry, Glenn is a great designer and his products are stable and without lashings of feedback to bring measured distortion down. To get genuinely better in all ways, involves far greater complexity (and VERY careful design so the complexity doesn't take the music away), higher costs and of course even more expensive bling casework to put it all in
Tear down these walls; Cut the ties that held me
Crying out at the top of my voice; Tell me now if you can hear me