I've been down that road, and I decided to buy the Strauss. I can't say it would have been the right decision for everyone, but I'm happy. I owned a pair of Beethovens for about a year and a half. I was satisfied with their performance as well, but had the upgrade bug. To be fair, I never tried a REL sub with the Beethovens, as I've rarely heard a speaker/sub combo that blends as well on music as a single speaker. I prefer to leave crossover design to the experts. I've heard many systems over the years that have a mid-bass suckout, or bloated bass added by a sub. I have also heard a couple of well set-up sat/sub combo's, but they are definitely in the minority, at least for music. I have a Mirage sub I use for HT only.
Anyway, about a year ago I bought a pair of Strauss. While I wouldn't say they blew the Beethovens away, they did make a nice improvement, but not really in the bass region. The midrange is where I noticed the biggest gain. It was a little richer, and fuller, than the Beethoven's. The high's were about the same, sweet and airy. The bass was deeper, but I don't think it is quite as tight as the Beethoven's, as I recall. Not flabby, but bigger. I can't really say better bass though. It's kind of a coin toss, I like the extra slam and thunder, but do wish it was as tight and punchy as the Beethovens. I guess that's the difference between (2) 7" drivers and (1) 10" driver.
The Strauss however uses the same ScanSpeak midrange drivers as the Mahler. These are fabulous drivers. This is where you'll notice the big improvement. Not only richer and fuller, but the soundstage grew as well. Wider and deeper. I'm happy I made the move to the Strauss, after one year with them, I have no upgrade plans anywhere in the forseeable future. My room is 14' wide X 24' long X 8.5' high.
So if you're only looking to improve bass performance, maybe the REL is the way to go. However, I believe the Strauss would be a better overall upgrade, YMMV.