@ozzy I was running two subs in stereo. It wasn't difficult to try the experiment. I took the feet off and used soft, grippy shelf liner and placed one on top of the other as close as possible. I pulled them out of the corner about 12" or so. This was NOT difficult.
The set up I use is... the bottom sub is crossover @ 40hz. The top sub is crossed slightly higher @60hz. Piggy back connection.
Why would this sound better??? Physics. When you 'couple' them properly you get an automatic boost of about 6db. That's alot. Free boost.
Why is this better? My bass is loud enough!! THIS is the important thing to understand. We've all seen the videos of the bass driver pulsing and extreme excursion pumping in and out.. This is the worst thing you could have! The more excursion = more distortion. The 6db boost requires you to turn DOWN the gain setting. This gives LESS excursion and less distortion. The bass becomes tighter, punchy and effortless. It's not working as hard.
Before stacking, my gain settings were around 1:00 on both subs. After stacking they are turned DOWN to around 10:30. Clearer, cleaner bass definition and plenty of punch with LESS excursion. My mid-bass is also improved due to the higher crossover on the upper sub.
THIS has been the best tweak to my system. Super easy and It cost nothing and took me an hour. Highly recommended.
Thanks to REL for bringing this to the forefront.