split sub signal out of preamp to use 2 subs?

The HGS-18 is perfectly fine for HT especially in a room your size. Two subs are usually required in a much larger room. My guess is that your problem has to do with the placement of your sub. Two sub will give you more placement headaches. If your sub is close to a front or side wall, I would suggest that you experiment by moving it away from each wall one direction at a time. If the HGS as a phase dial, you should experiment with that if the sub ends up way out in front or behind the left/right.

Two subs, double the trouble. Worth it if you need them though but I don'e think you do. Good luck.

While I can't agree that two small subs are better than one
large one, it's simple to put a "Y" cable on the sub output.
Using "Y" cables is pretty much the same as having two outputs because the outputs are usually just parrelled on the inside of the preamp.
As far as the subs go, of course you can get very good small subs but it's easier to get deep bass out of a large cabinet and driver. Most smaller designs need very large amps to move the same amount of air.

It should not hurt to fork the sub-out. Nobody will highly recommend doing so but should not do much sonically.

My pre has RCA and XLR main outs. Therefore, the L&R RCA outs are connected to the sub's L&R ins. The XLR main outs go to my amps XLR ins.

More than likely, your Velodyne 18 inch would be better than most all other smaller subs in the same price range.

Many will say, (I think without critically listening first or A/B comparing) that a smaller sub is faster and tighter. In the HGS 18 price range that most likely would not be true. It may seem tighter because very, very few smaller subs can reach as low as your sub, therefore, what it can reproduce may give the impression that it's really tight when in fact you're hearing more silence because it simply can't do what it should be doing. And that is reproducing the lowest frequencies say 24Hz down to below 18Hz.

For example, the NHT Sub Two has two 10 inch woofers and a 750 wpc Carver amp and NHT calls it musical. I've heard it in two private homes (one HT and one music) and tried to correct and configure it. Well, let's just say even though it retails for $1200, I wouldn't buy one for $50 unless I wanted to try a DIY home project.

It was not tight in the least and with one of my cd's it simply could not reproduce the lowest of the lows as it congested, compressed, and muffled horribly.

I have not listened to that many subs but with the ones I have heard and read about most of the smaller subs peter out at about 30Hz anyway. Well a good woofer goes that low. I'm not even sure why anyone would call that a subwoofer.

I have full range Aerial Acoustic model 10T's with a 10.7" woofer faithfully reproducing down to about 25Hz. Below that I'm using a pretty tight Triad Platinum 18 inch sub and thoroughly enjoy it. The volume is barely on with mine also. Perhaps it's set a 1 or 1:30 (12 being off or full blast).

Before getting rid of what you have and buying a smaller one, I would recommend that you do some serious A/B comparisons in your home first.

There is of course the Aerial SW12 which has a 12 inch woofer and is flat to below 20Hz. But it also costs a min of $4500. So there are a few smaller subs out there that are very good. But I'm guessing that only when you pay substantially more than what you paid for your HGS 18 inch sub.

Based on what you are trying to do, I would say one good quality sub is better than two cheap ones.

Two subs are clearly better than one, only if you were able to run stereo subs (each sub playing a separate channel). You could run stereo subs, but you would have to run them off the speaker terminals, with a sub that can handle such a connection (high pass). You would lose the separate sub channel on your HT setup. Such a setup is great for music.

It won't be stereo if he has to use a "Y" cable.

Thanks to all of you for your helpful responses.