Comparing products, even what you think may be dissimilar ones is always a good idea. It's great practice in listening for specific differences between things, narrowing things down, taking careful measurements, etc. Plus it gives you something different to listen to that you might not expect.
What you *thought* before was how it is supposed to be, might be totally different if you listen or see a new product.
This is not usually the so much the case with subs, but a good comparison on both music and bassy action films is a good thing to do.
And make sure you be patient and spend a long time with each, it's the best way to do auditions. I prefer it over the quick passage-switch to sub B, same passage, change, switch back, etc.
All you tend to hear are the big differences between them. By listening to a speaker for a longer period of time, you aren't just getting the startling differences, liks "gee, where'd all the bass go" or "gee listen to all that new detail," but you get the whole package, and begin to hear other differences that would have been glossed over for the larger differences. That has more to do with regular speakers, subs are usually less complicated things, but the same can be said.
