What tubes do you have in the MPX? Old Stock, or New Russian Tubes? Brand? I've found quite by accident that tubes with sections that have pretty much equal readings (gm and emissions) will have less problems than those with a good section and a weak section in V100 and V101. This goes for both new Russian Tubes (not too likely unless from the factory that way) and old stock domestic and euro tubes. Like Dave said, Russian tubes are more likely to have problems in the MPX, especially V100 and V101.(Russian tubes also have problems in the Phase Inverter section so using a Domestic or European old stock tube is a good idea here. Make sure the sections read close to each other.) What I do is to put Domestic or Euro old stock tubes with fairly matched sections in V100 and V101, and a domestic (if I have any left over at that point) in V102 (which is the MPX Amplifier) or a highly matched, very good testing (gm and emission) Russian Tube. As an example of a somewhat similar problem; I have a Sansui 1000A which has the following tube line up in the MPX. 6AQ8, 6BL8, and a 12AU7. It's aligned as is the tuner. Now for the fun part. A 6DT8 is the sub for the 6AQ8. The 1000A uses 3 of the 6AQ8. 1 on the Front End of the tuner(Mixer/Osc), one as the Muting Amp, and one in the MPX (Mpx Amp and Indicator Amp). The 6BL8 is the 19kHz synchro and Doubler, while the 12au7 is the "DUAL, OUT & De-Emphasis). I put Brand New 6DT8's in all 3 positions and lost STEREO. So I put a crappy reading 6AQ8(both near minimums and mismatched by 2000mhos) back in the MPX and I got Stereo again. The other two posit's are fine with 6DT8's. Matter of fact I use the 6DT8 in most all places that call for a 6AQ8 (cost is $3-$4 for the 6DT8 vs. $15.-$25. for the 6AQ8) in all of my tuners, or wherever a 6aq8 is called for. I haven't had to replace one 6dt8 in 10 years on any of the units.