What a great encoder, just perfect. Have used MPC for awhile, and concluded
that this is the best lossy codec avabiable.
Some "issues" is there,
A. Seems to me that mppenc have a issue regarding low frequency.
(is that right.?)
B. encspot reports input frequency to be 32khz or less, while mp3, mp4 have 44100.
(is this a encspot thing or do mppenc have a upper limit ?, in case, i just don`t get it.
mpc handles the upper freq. range much better than others, at reasonable bitrate.)

Some more questions:
1. would future mpc builds have a dll.? (ACM would also be great.)
2. Is there ever been concidered to have a hybrid mode, to mppenc.?
(sort of the correction file in wavepack.)

A) Don't think so, at least haven't heard about it... It's doubtful it has issues with LF though...
B) Haven't used Encspot yet, but you seem to be referring to the "sampling frequency"... It's almost always 44.1Khz unless you change it for some reason... Maybe your MPCs were resampled at a lower frequency... Btw, it's not the same as the highest frequency available (or upper limit) in the MPCs, although they are related... Sampling freq divided by 2 equals the highest freq...

No, this is not a known issue. It is probably because of an equalizer setting in your player, that would affect MPC and not WAVs. This is a known issue
If the setup is ok, and you find the bass not to be properly encoded, then isolate a sample, and perform a blind test (see how in the FAQ or the Wiki). It is necessary for the issue to be taken into consideration (see why in the TOS)