I hope this is the right place to be asking this. I have a majority MP3 collection which I have ReplayGained using MP3Gain to the loudest non-clipping volume I can get album-based for playing on the computer through WinAmp.

That works fine for me, but, I've now downloaded an album I really like but which was only available in MPC format. It is considerably louder than the rest of my MP3 collection. I'd like to adjust its volume downward to match the album volumes of the rest of my MP3 collection but for reasons of retaining as much quality as possible I don't want to transcode the files.

I've downloaded ReplayGain for MusePack and Speek's frontend for it and have been playing about with them trying to understand how they work, but I need help. I can't figure out how to compare the info I get from ReplayGain for MusePack with the info I get from MP3Gain. With MP3Gain you input a dB value you want and the program makes the adjustments as necessary to achieve that. With the MPC version you seem to have to input the adjustments you want. But how can I tell whether that will match up with the volume levels I already have for the MP3s?

I don't think there are any MPC decoders that don't support replaygain. Assuming you're using winamp, have you tried turning on the replaygain feature in it's mpc decoder ?

In the MPC replaygain (just like the one in foobar) the target loudness is fixed.
Yes, you can manipulate by setting the values but it is a bit besides the idea of Replaygain.
If you use Winamp be sure to set the level in the MPC decoder to K-14 (as mentioned in the recommended settings).

Oh and, it will only have an effect if you use a player/decoder that supports replaygain, unlike MP3gain which also adjusts the gain structure in the mp3's.

A bit OT, but I'm wondering... say I have a recording that, if adjusted to 89dB, still clips. What does ReplayGain do with such a recording?

ReplayGain is just a value And it might be so that the loudest parts would clip if applied.
It can even be that on e.g. a quiet track with some peaks that do not clip, ReplayGain is calculated as a positive value (like + 2.00dB) and that would make it clip (This is not uncommon on older recordings or classical). Replaygain is not clip protection persé.

As it's up to the player to use it, a smart player (can I mention foobar2000?) can use the ReplayGain information (not only gain but also peak values) to avoid clipping. In foobar you can set this in the Options->Playback "use peak info to scale down tracks that still clip after applying replaygain"

Yeah sorry. Foobar2000's respone was what I was asking about. Thanks.