The issue of whether Axiom or SVS is a better value, isn't as important here. I don't have experience with mixing two subs of different brands, but from what others have said and articles I have read on the subject, I think that it is quite important to match them as close as possible. Like for example if you wanted to get an ep600 or ep800, I think that they would blend well together and definitely smooth out the bass from 20Hz up to the crossover well. When I go with multiple subs I am definitely going to make sure that they match. Right now I have an Outlaw LFM1. Since releasing it they have replaced the sub with an LFM1-Plus and LFM1-EX. I imagine either the Plus or EX would blend very well with the original Outlaw sub since they have same driver, cabinet volume (unless I'm mistaken) and nearly the same amp. They only slightly upgraded the amp and added some EQ so that you could plug ports and get a lower frequency response.
I too am a little bit torn though, since the SVS sub you mentioned seems like a slightly better sub to me than the Outlaw LFM1-EX. But, if I'd go with the SVS I'd buy them as a pair and either sell the Outlaw sub, or use it in another room (you can't have too many subs). Anyways, you may be able to do a similar thing. If you really want to go with the SVS subs, then buy them as a pair and sell your Axiom sub. What you get from the Axiom sub should probably make it so that you only spend about $700 or $800 like you wanted to in the first place. You would still have more headroom than before and if you place them properly the bass should be nice and smooth at every seat (which is the whole point anyways).
