your opinion on subwoofers in two channel

I have a pair of PMC GB1s 29Hz-25kHz. Heaven in the mids and highs, just need a tad more low end. I have limited space 15x11 and limited budget, 700.00. What specs in a sub should I be looking at? This is something I do not understand when reading the many threads on 2 ch sub matching...

The main problem with subs is that they are extremely dificult to locate, you can be playing around with them for the longest time, and when you think everything is in order, you turn off the sub while playing music and you realize how much music is being "blocked" by the sub, a lot of information on bass and midbass that is being cancelled by the sub in order to get the whuum dull sound of the subwoofer.
I will try to draw an image of what I am saying, Imagine your sound is like a graph with crests and valleys, when you add a sub you get a line in the center (lower maybe) and from that line you have mids and highs going up and lows going down, but everything around the line is blurry, When I finally located the sub in a "good" position I would get deeper sound, the line was erased and the valley seemed now deeper (a good thing) than with no subwoofer, but beware, I had masking tape all around the subwoofer marking 1/8" movements!!!
My opinion today is that a subwoofer should be crossed higher than 100 hz so you can listen clearly to what they are doing, and be aware if they are messing everything up.

If you are willing to get into this, get subs, with the price a good sub is going for, and the terrible resale value they have, I would recommend changing main speakers for more bass capable ones...

If you use subs the smartest thing you can do is make bass traps for each corner. This will help considerably. No modal ringing plus it flattens the response. Without them bass normally isn't nearly as musical in my experience.

Electronic equalization with a sub is also good as the bass region is where the very large dips and peaks occur. Digital equalization like the Behringer DEQ 2496 will help alot especially if you cannot install bass traps for whatever reason.

Without all of the bass music doesn't have it's foundation. To my ears something is obviously missing and I find it distracting. Honest bass can make a good system much better.