Vandersteens and subwoofers other than the 2Wq?

With the Vandersteen's, I would go with the 2Wq. It is designed to mate perfectly. Why did the REL stick with you more? You really shouldn't "Hear" a sub. It shouldn't increase the loudness, it should only allow for the lowest octaves to be reproduced. Setup will play a bigger part than anything.
The REL is a nice sub and should mate with the Vandersteen's if you want it to. What I would listen for is does the tonal qualities of the REL reflect those of the Vandersteen 2Ce's. Also, the Vandersteen sub is 300 watts vs 100 for the REL and uses 3-8" drivers instead of a single 10". Also, with the Vandersteen Feed Forward Error Correction using a 80 hz crossover, you cannot dynamically restrict it and/or cause distortion.
With all that said, the choice is ultimately yours and YOUR listening preferences. Need to try both and see what works for you using the 2Ce's with each sub.

There has been a lot of discussion on A-gon over the past 18 months or so about the Vandersteen subwoofers (2Wq and V2W). If you have not perused the A-gon archives, I think you should do so -- there is a lot of information that will be informative for you.

A 2WQ does sound quite good with optimal placement. However, the Vandersteen's 6db/octave crossovers do not allow it to disappear into the soundstage as well as the steeper (and lower) crossovers of the Rels and ACI (Titan II LE and Force). This is especially noticeable when the subwoofer cannot be most optimally situated.

As well as allowing the more flexible placement options that my room requires, my ACI Titan II LE also seems to go louder and deeper than the Vandersteen 2WQ, and (IMHO) it is also a fine-looking piece of furniture (it functions as an attractive end table in my living room).

Setup is also quite flexible in that it can be run Rel-style (with the main speakers running full-range and the subwoofer crossed over below that) or quasi-Vandersteen style, using ACI's 12 db/octave highpass filters between the preamplifier and the amplifier that drives the mains.

I've used both methods to seamlessly integrate my Vandersteen 1C's with my Titan II LE. Each method has its advantages in theory and in practice, but recently I'm gravitating toward the highpass filter method, because it usually sounds a bit less congested and clearer in the lows and mids. It also provides me with extra peace of mind when I play the occaisional explosive movie.

Given my strong Vandersteen orientation, I probably would have looked no further than a 2WQ, if I had been able to get it to work okay in my room. I am very glad that I checked out ACI.

I have the REL Stadium III. It is a wonderful sub-woofer. However Sek, Sdcampbell and others have identified something very important. The Vandersteens, in particular, tend to be congested (to my ears) in the mids. The highpass filter relieves the burden placed upon the mains to reproduce low frequencies and tends to clear up their presentation, while passing the burden for VLF to the sub. This enhanced clarity in the mids is, in my opinion, the primary gain of using a sub with these speakers; not low frequency extension of the mains.