Rear Speaker Advice needed

Consider the Proac 1SCs. They should be a decent sonic match for your Studio 140s. While they lack the deepest bass, presumably you've got that covered with the mains or a subwoofer.

I second Jameswei ....you should stick with the same manufacturer - your best match is actually the Studio 110...as it has matching drive units!

1SC's would probably be the next best choice but poor in comparison due to their reduced dynamics relative to your big 140's. It is impossible to achieve balance and a decent HT sound if you mismatch mains and surround speakers too much. (You end up with a system that only sounds right at some volume levels and not others...and if you use completely different speakers form different manufacturers then all I can say is, well, you have a dogs breakfast of an HT system and it will sound that way too)

Yes, the 140s with the down firing ports nicely covers the bass. And I intend on using only the mains for music listening, so the rears would just be for home theater.

But the 1SCs look like they're a better speaker than I need. I would think the Studio 100s would be a better match and they are a little cheaper. I'm also thinking about looking for Dynaudio Focus 140s or 110s as I wouldn't think that brand matching should matter too much, hence why I'm wondering what other company lines are comparable in terms of quality and sound to my mains.

I agree that the Focus line occupies about the same position in the Dynaudio range that the Studios occupy in the Proac range. Both are highly regarded speaker makers with challengers to the "best" speaker title at the tops of their ranges.

Both Shadorne and I believe it would be better to stay with the same manufacturer, i.e., Proac. Given that you have decided to set this aside, I would recommend the next step to be auditioning likely candidates. Presumably, you already know the "sound" of your Studio 140s quite well -- do target the best match during your auditioning. You appear to hold budget discipline in high priority for this decision. Accordingly, likely candidates for auditioning can be selected by price point proximity, after adjusting for used versus new pricing. Good luck.