All the instruments sound great, and the performances are awesome. It's a very good mix overall. I do agree that the bass is the only real problematic thing. It sounds nice when it's audible, but it seems to be kind of coming in and out a bit, phrase by phrase. One phrase will be nice and loud, and the next will be much less audible. It seems as though some automation could be used to even it out before adding any compression.
Regarding the arrangement, in lieu of a bridge or contrasting section, it would be nice if there were at least one really obvious solo section, you know? Like ... "Here's the cello solo!" And maybe a few things drop out to thin out the texture at first to signal it's happening. As it is now, it doesn't really seem as though anything really takes center stage the whole time. It kind of sounds like a bunch of people jamming (very well!) along and looking at each other as if to say, "Ok ... what next?"