Did you take a few familiar DVDs of the sort that get plenty of discussion on here. How about a CD with a good rangy and fast bass line. It's easier to demo if you can ignore the program material and focus in on what matters. Unfamiliar material won't help, in this respect.
If the REL was a better, deeper sub, could you tell? If you could, is it worth the extra? Generally speaking, if you can't hear the difference, then why spend more?
However, unless it's in a room of very similar size/dimensions to yours, then the demo isn't particularly useful. The room in which the sub will eventually live, is a dominating factor in it's balance, volume capabilities and integration. The physical makeup of the room can also have a large bearing on the performance. Stud walls, for instance, will absorb a lot of bass and could leave a small, or underpowered, sub strugglinginspite of the room not being that big.
If you can't get a home demo, then you may as well buy on the strength of the specs/price/one you like the look of. Dealer experience will help, but unless they know your room at least in passing, then it's no more valid than a lot of the advice on here.
If your local dealers won't offer that, then the AV Sales 45 day money back offer, will.
For all the pitfalls associated with buying online, it is a little surprising, given the much vaunted specialist dealer service, that it's the only easy way of finding out what you really need to know. This is, however, the lesson of finding a dealer you like and sticking with them. They're far more likely to allow a few days with a sub if they know you, of have a swipe of your credit card.
Russell