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Recently, I completed a pair of Mark IIIs using the Poseidon boards and am delighted with the results! I have read a lot about using Russian PIO caps in tube amps, so my question is, would I hear a noticeable improvement in sound quality if I substitute Russian PIO caps for the Orange Drops on these boards?

A lot of this is personal opinion , but the Poseidon is very natural sounding with caps like the orange drops , PIO caps tend to roll off the top end and make it fuzzy sounding , if an amp is bright sounding PIO's will mask it .

If you use the green "kommie kaps", check them with your DVM for leakage / resistance before installing them, I have found a couple of them, which had
high resistance leakage, and that threw the control grid bias too "positive", and that made the affected tube draw seriously excessive current.

/ed B