Nearly impossible to comment on the mastering without hearing the unmastered mix. But in this case, there really is no mix except for the room. So, I... Uh...
Anyway -
That said, if this is just a stereo room recording, you've accomplished a pretty decent sounding stereo room recording. Sounds like a fairly out-of-control space though... Not necessarily in a bad way - and not necessarily a bad-sounding space - But not a typical 'intimate' jazz recording either. Soundcloud is doing a number on your top end, but I'm assuming that's just Soundcloud.
On to other things -- Next time, try SDC's. Even a Zoom (H4? Something like that). For straight-from-room recordings, they do quite a decent job picking up a surprising amount of detail and rejecting a surprising amount of the boxiness in many spaces.