While I don't disagree that a sub might be a good first upgrade, I do see a couple of points to consider,
- While I'm not really familar with the HK3490 (quick look at the manual only) I don't believe it'll relieve the bookshelves from producing bass, it has no management (x-over) I can see for that. While the sub will obviously produce better bass and dig deeper, the little 153's will still be doing full frequency duty.
- Not sure if the last list of speakers the OP posted would be a lateral move, some might provide a fairly good sound upgrade but, as mentioned already, auditioning is the only true test to see if the upgrade is worth it.
Since this is primarily a music system I'll throw out another option with the thought of trying to keep a budget in mind. You could drop the sub/bookshelf combo and get some towers instead. I normally switch between 2.0 and 2.1 when listening to music, depending on genre/mood, but even when in 2.1 mode for music I lower the sub down 3-6db from my HT settings. In any case, it's just more food for thought.
Steve