I tend to be somewhat skeptical of the need for isolation cones for something like a digital amp with no moving parts or microphonic issues like tubes. I have an S300 (and enjoy it very much). My room is such that I've tried the equipment in different spots, including one where the amp was in a different room from the speakers (with no change in speaker cable length.) There was no difference for me, even with the amp physically in another room, so I wouldn't think isolation cones/feet would make much difference in midrange quality. If that's the case, your best bet is to simply go with what you like.
Your mileage may vary.
The problem in evaluating changes such as this is there are so many variables and a lack of set reference points. It may be that the bass and and high extension of the S300 is simply strong enough that the midrange doesn't sound as forward. My reference points for a stereo are live acoustic instruments. If those are "right" then I'm pretty happy even though I know some recordings might sound better on another system. If your reference point is pop/rock or other music that depends heavily on amplified instruments and heavy studio processing, it can be far more difficult to find a "neutral" reference point.
Your mileage may vary.
The problem in evaluating changes such as this is there are so many variables and a lack of set reference points. It may be that the bass and and high extension of the S300 is simply strong enough that the midrange doesn't sound as forward. My reference points for a stereo are live acoustic instruments. If those are "right" then I'm pretty happy even though I know some recordings might sound better on another system. If your reference point is pop/rock or other music that depends heavily on amplified instruments and heavy studio processing, it can be far more difficult to find a "neutral" reference point.