Yes I have. I have a classic PAS2 preamp which last year had one channel low, not caused by tubes. So I bought a disco mixer with 4558 op amps to get by. I replaced all the paper caps in the high gain stage of the PAS2 with plastic film, and of course it was the last one that had the burned wax shell (hidden on the bottom, of course) that had caused it to deteriorate. I then changed the PAS2 plate resistors to metal film, changed the H.V. electrolytic cap, scrubbed and scrubbed to eliminate arcovers on the PCB's, and it now sounded better than the op amp mixer on both my transistor amps, the ST120 with DJoffe bias mod, and the Peavey CS800S. A little less hiss than previously idling, and much less hiss than the disco mixer. However, the volume pot on it is stuck; a new Stereo Cost Cutters OEM pot from the Dynakit bankruptcy reject room. Has to be turned with vice-grip plers. Haven't found any stereo 250k log replacements with the loudness track.
So I bought some 33078 op amps for the disco mixer, which had great slider pots. Quieted the hiss right down, but it had maybe -40db of hum. Long war on hum ensued, grim details on
http://www.diyaudio.com/forums/anal...improving-disco-mixer-mid-fi-performance.html. At the end the disco mixer has equivalent hiss, equivalent lack of distortion on top octave steinway piano, and a tiny bit more hum than the PAS2. Have to stand 20 cm from speakers to hear the hum. Have a question going on analog line level thread about changine the RIAA curve parts on the disco mixer for even less hiss (kitchen sink preamp post).
Both Pas2 and Disco mixer sound the same into a dynakit ST70 with new output tubes, but ST70 mid range piano is honkier than either transistor amp. Top & bottom octaves sound nice. Looking at a triple twin triode input board for the ST70, but the triodeelectronics.com version is designed for EF86's of which they sell only a poor eastern block copy. (6pi something tube is what I got from them) I'm not into etching PCB's anymore.