That sounds pretty bad. I've bought these no-name ones and I don't have any problem while in the same room as the N900, they work pretty good in adjacent rooms as well, if anyone is looking for a cheap pair I'd recommend them since their sound quality is also decent. I've also bought some other cheap ones which haven't been so good reception wise, including some S9 knock-offs from ebay which I wasn't expecting to be particularly good but they are actually so bad as to be unusable. Recently I've also bought the Sony Ericsson MW600 headset/headphones and the reception with them has been excellent, much better than the previous mentioned headphones, I can practically go all over my house with very few dropouts, they do give the occasional issue with pairing though.
A while ago (around new years) I upgraded from kernel-power v45 to the power kernel v46 with the injection drivers and then started having problems with bluetooth audio, initially thinking it was the modified driver causing a problem switched to titan's kernel-power v46 and the problem still persisted, but I didn't do much testing to confirm it because my USB port broke shortly after that, and since getting a new (second hand) N900 and my old one back I still haven't got round to re-installing kernel-power. So if you are using kernel-power v46, try downgrading (or switching to stock).
As a disclaimer from what I previously said, I still get occasional wifi interference even with the MW600s, but mostly only when it is scanning for wifi networks.