Either will be better than the Solens, and don't worry about .3 mf.
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I agree that the .3 uF is no big deal, too small a cap will roll off the bottom end so in this case bigger is better.
I don't necessarily agree that either will be better than the Solens. I would assume that if you can hear a difference at all, it could possibly be for the worse. Either may be more expensive and either may enjoy a better reputation in the DIY community, but that doesn't mean that either will automatically sound better in this particular amplifier. There are very few absolutes in this hobby and changing the components in a well designed piece of equipment can lead to unexpected results.
I agree / disagree with both Sportinlife and Herman. Personally, i think that either the Infinicap or Auricap is a more neutral and better sounding cap than the Solen. Which one you like better in that specific piece of gear or your system is a personal choice. My friend Lou is playing with both of these caps and thinks that they both have strengths and weaknesses. I'm sure that he's right.
Having said that, Herman brings up a point about "misadventures in upgrading & modifying". Quite honestly, some circuits sound worse when you change components. Some units are voiced to sound a certain way with certain parts. Changing those parts may reveal / uncover other flaws or sonic characteristics in that component or the rest of your system may have. If the goal of the designer was to obtain a high level of accuracy and musicality, upgrading passive parts "should" only offer you more of a good thing. If the designers goal was to produce a product that had a specific sonic signature and you do decide to change that design, you could be playing with dynamite. That is why i ALWAYS stress taking notes prior to dis-assembly and labeling both the parts and polarity / direction of each component removed. If need be, you can always go back and re-install what was already there EXACTLY how it was from the factory.
Having said all of that, my experience is that using better parts results in a better system. It's just a matter of finding out what is actually a better part when comparing Brand X caps, resistors, wire, etc... to Brands Y & Z. Obviously, opinions and experience vary here. Sean