Maybe unobtrusive needs defining… Aside from small, which is hardly the way to go if you want more/better bass, possibilities include narrow frontage, or maybe available in a colour/finish that blends easily with room decor, or perhaps speakers that fit very close to the wall, or even tuck right into the corner. PMC Fact 12s might be meet the first (and secondhand would be in budget (secondhand maximises buying power). An example of a speaker fitting the second, though way over budget even secondhand so not a real suggestion, is Tannoy Westminster Royal IF you have a very large room with period decor such as wood panelling, where it would likely blend in despite rather generous proportions. Otherwise for second there is any number of speakers with nice wood veneers, and a good few available in black or white, and a small number in others colours though that would limit your choice. Naim SL2s might fit the third, but doubtful if bass is adequate, some other speakers can do well like that as well, though often depends a lot on rest of room. Finally one speaker that suggests itself for the fourth approach is the corner horn.
If you consider adding a subwoofer (or two) I would also recommend that you take a look at Rel. I have a couple of the Rel S series in my system and they do what it says on the tin. They are specifically designed for stereo systems (rather than home theater) and so seem integrate very well and are fast.