I am an unabashed Proac fan. I love Stewart Tylers emphasis on midrange and find them to be unfailingly natural sounding from lower midrange through upper midrange. I am not compelled to have ultimate high frequency extension so I am completely satisfied with the soft dome tweeters and I think that the ribbon tweeters on current models positively extend the highs for those that feel this is really important. I have owned: Response 3, 2.5 and currently own Response 5. (also Response CC1, CC2 center channels and ERB1 subwoofer). A very close friend of mine owns D30R.
Regarding bass response. The traditional Response ProAcs (which were normally rear ported) are quite sensitive to placement in the room, especially to the back wall. My friend’s experience with the D30R which is bottom/side firing ported was very sensitive to being placed near a corner which excited a significant room issue right around 40-45 Hz. With his room layout, it was just unreasonable to move them far enough away from the corner to solve. When we did move them temporarily out, the issues were nearly completely eliminated. In his case, because of the room layout, he had to move to a non-ported speaker (YG Carmel II) to satisfactorily solve this problem. Of course this speaker also costs 3x!
My experience is that with ProAcs, 3’ or more is really ideal on all sides. As other posters have indicated, there are tradeoffs with every design. The design has much in common with my previous Response 3. The tricks that Tyler uses to provide robust bass with such small enclosures and drivers trade off some definition in the bass region. The gain of these smallish drivers and enclosures is magical midrange and fantastic imaging. Even 6 more inches of "air" on front and sides may mitigate your issues. Keep playing with positioning. Perhaps look at getting some room analyzing equipment (like XTZ room analyzer). Fractions of an inch can matter with this speaker (maybe with all speakers...). When placed correctly, bass response on the D48s should be exceptional -- well balanced between extension and control but from a musicality standpoint, the overall sound presentation should be amazing. Good luck!