This is some good stuff. You have a lot of great ideas and have put them together well in general. I'm going to print them out and try them.
One thing that I noticed, and others have commented on to some extent, is that they are all very bass heavy. While for specific purposes that is o.k. if performed as a set it does get a little annoying. Even with single notes in the bass, you have to be very careful with pedaling (which problem showed up in one of the slower ones where the pedals weren't clean.) One thing to consider if you want to keep a strong bass presence is to transpose them up a 3rd or 5th. This will keep the bass strong in in the bass range yet clean up some of the issues.
Also, intervals other than octave or 5th much below the C below mid C might as well be minor seconds because of the nature of the overtones, as Eddy brought up. This is fine for special effects, such as percussive type sounds that occur in no. 1 and to me work ok.
The other thing in no. 1 that you might consider is balancing all of that lowness with your lyrical melody in the meno mosso section working up an octave higher, maybe in octaves.
No. 2 I think would be successful in the hands of a human. To me it needs a slower tempo and to feel less metrically precise, which I am sure is an issue of working on the computer and relying on it for playback. You have it at about quarter = 60, It would probably be better around 50 or so, more of a lento than an andante and would give more of "melancholique" spirit (Lento [or in French "lent"] would also make more sense as a term to combine with "melancholique". It is also one that you might consider trying it about a minor 3rd higher, making your lowest note the F below the C below bass clef instead of the D.
No. 3 is pure BeBop (a shoo bop a bop --- oh well, I'll stop singing). Here again, I would like to hear the right hand venture further up the keyboard. In this piece your L.H. has some wonderful nearly 4 octave sweeps in a single gesture. The R.H. never really ventures that far in its fast moments. Similar sweeps upward or downward from the top would add balance and give added import to that short lyrical that is firmly grounded in the center of the piano -- the voice of reason trying to reign in two opposing forces that have no center to their thought.
no. 4 This is another one that I think might be more successful up a step or two.
no. 5 This is bottom heavy. You have a great bass melody that looses clarity because of its range. An octave higher or a transposition of the whole piece would be beneficial. I also believe that a slower, more fluid tempo might be more in keeping with the "Lamentoso" spirit (again, I understand that the computer doesn't understand espressivo like a human does). Another thing that you might consider (particularly if you use a slower tempo) is to let the accompaniment 16ths rhythmically modulate, possibly into sextuplets and then into 32nds. This would add more intensity without actually increasing the tempo.
no. 6 This is another one that would possibly be more successful transpose up a little so that it isn't so bottom heavy and also allow the R.H. to venture further away from the center.
As I said at the start, there is good stuff here. Also, from what I hear and what I see without actually sitting at the piano, they actually sound pianistic with piano type figures and passages. Are you a pianist? Often, composers not totally familiar with a particular instrument will create things that come off more as transcriptions from another instrument that the composer may be more familiar with -- such as writing for a violin but actually writing more flute-like passages. Even some well know composers did this. I believe that it was Bruckner(? I may have the wrong one) wrote for orchestra (and wonderful stuff) as if it was a transcription from a romantic organ piece since he (or at least the one I'm thinking of) was an organist first. If you are not a pianist by nature, you appear to have a good basic understanding of the instrument (other than the bass thing
Keep up the good work and I look forward to hearing and seeing more of you compositions.