Experience with stacked subwoofers?

A Ripole Subwoofer might be the one to investigate. 

Everyone hears differently and has different values regarding audio purchases. I've heard several systems with REL 6-packs. I didn't like the sound of any of the systems I've listened to with 6-packs. I actually own dual REL subs, so I'm not being a REL hater.

Thanks for all these responses. I have two Rel t7s in stereo in my current set up, which were one of the greatest improvements made. In my own personal experience, I have found subwoofer replacement and even toe-in to make a significant impact on the quality of the base that I hear from the listening position. Not just the amount of bass, but the ability to hear distinct differences in the notes from one to the next. Having experienced a dramatic difference with that, I’m imagining that adding a height dimension would also be significant. So I’m intrigued with the idea of stacking three subwoofers, each tuned to accomplish something slightly different. 

A good friend of mine has a pair of GR Research, Double Trouble open baffle subs. 

One stack per channel.

Probably the best sounding subs I’ve heard.

Well, tomorrow is my day to try dialing in my REL S/510 four-pack. I’ve watched John Hunter’s Youtube about doing that setup for a six-pack. When my dealer delivers my last pair they will send their expert on setup to help get me sorted, so I’ll see how close I get to what they think is good, at least up to the point of working on fine tuning. I even have the Sneakers Track 4 audio they recommend , ready to go on Qobuz.