My focus is on the source side so anything to improve that is should be more important than downstream (unless you have a weak link, like the absence of a DAC in your case). Clean sound comes from clean power/signal and what you hear should not be cluttered by too many or strong indirect reflections. In your case I would certainly invest in a good power conditioner as that would benefit all items in your system and subsequently, power cables and interconnect upgrades. Cheap tweaks in isolation (a must for source devices) and vibration control (amps and speakers) should definitely be considered, there are numerous entries here on the Gon about that. Room treatment can be more difficult as to find a compromise between room aesthetics, according to your partner's opinion, and room acoustics, unless you have a dedicated listening room. My practical advice for the first grand (you can never stop, that's the fun part!) would be:
1.) buy an used power conditioner for $300 (heck even a cheapo Belkin PF30 or PF60 is better than nothing) and replace the wall sockets with hospital grade ones (e.g. Hubble or PS Audio)
2.) an used DAC for $600 (many contenders)
3.) isolation tweaks (Herbie's Tenderfoot, Vibrapods which are very affordable).
Simao, we have not heard back from you so maybe you could sum up what you have done or going to do and we could go into the second iteration of advice/comments.
