Subwoofer placement matters. You can at least double the sub's output by putting it in the corner like you planned, so please try a few things before you spend any more money on subwoofage.
Have you heard of the "sub crawl?" Place the subwoofer in your seat, then crawl around at ground level to find where the bass sounds best. When you trade places, the sound should be the same.
I'm bothered by your comment about sub volume. Unless the input is active, you should hear nothing but a little hiss (very little in a good amp) even at max volume, in the absence of signal. How are you connecting it? What's your AVR?
You have a computer; does it have a sound card? If so, run a line level signal to the sub's line level input. You may need a patch cord to convert plug format, but there are a lot of freeware signal generators out there. HEre's what I use (ignore measurement for now, just use signal generator):
You can send pure sine wave signals, or white/pink noise. I'd use pink noise for the crawl, and sine waves to test limits and for room mode resonances. Add an SPL meter like the Radio Shack unit and you can determine SPLs and even do measurements (it'll connect to the PC as well).
If external signal inputs to the sub result in a large difference in response compared with HT use, there's something odd in the connections or AVR set-up. If the sub won't behave with external signals either, you may have a defective sub. The only thing a larger sub can always do better is play louder and dig deeper. Make sure you've seen the limits of the current unit before spending money on a larger one.