2 REL Strata Subs and Mono amps

Zormi, I would like to know what your main speakers are, because I believe you have a totally different problem to Jdolgin, and it hasn't been addressed: you have an active crossover in your system (Krell Phantom's crossover board option) and RELs aren't designed for this; they are supposed to 'augment' the main speakers from below their natural roll-off frequency, without the use of a crossover like the Phantom's. If this is the case (unless you already purchased the subwoofers), you don't need REL Gibraltars as there are many good and less expensive alternatives, some of them with room correction built-in. On top of that, if your main speakers are full-range floorstanders, you don't need the Phantom crossover to use the RELs; if they aren't full-range and depending on their bass response, you can go either way.

Zormi, I normally don't suggest components unless the poster asks or presents alternatives himself. In your case, I would suggest giving a good listening to the Confidence C1 IIs at about half the Diablo's price. In my humble opinion they are among the best standmounts ever, for several reasons: deep and controlled bass enough even for medium-sized rooms, a soundstage that includes height reproduction, something rare among non-tower speakers (even Vandersteen 5s and 7s, as well as highly rated standmounts such as Raidhos don't do it), and all around sound as good as anything at two or three times the price. The C1 IIs definitely don't need a crossover, just augment their already impressive bass with your RELs at about 30Hz or a bit below.

By the way, I checked the Gibraltar manual online and on page 12 it explains the way to connect to differential amplifiers such as your Krells, basically to connect the black Speakon wire to a chassis screw as was explained by others here. That's also how Nelson Pass suggest doing it with his balanced amplifiers, and he provides another set of ground posts just for that.

The C1 IIs sound better than the Mk Is, but I would take that Diablo bargain from your dealer in two seconds flat!

By the way, I used a Storm III for many years with several speakers, including B&W Matrix 805s, 805s and 804s. One thing most people don't know about RELs is that they can improve the bass of almost any speaker, but that's not their main advantage; it is the way they improve the soundstage and the general sound of the main speakers because of the ambient clues hidden in the low frequencies and the way they 'come from underneath' the main speakers.

Enjoy the music!

Good that you say that, I thought about it but at the end didn't write anything. I used my REL Storms with mono and stereo amps at different times. The example on pag. 12 (top) of the REL manual is of course for a stereo amplifier; for mono amplifiers, use the pag. 12 (bottom) example and just imagine the gray amplifier divided by a vertical line through the center, so that you have two separate mono amplifiers, a left one and a right one. The potential grounding problem still stands, so it's better to ask Krell or get your dealer to do it.

Zormi, I forgot to add that you should copy the grounding scheme of the pag. 12 (top) example for the (bottom) example, of course. I think the chassis screw grounding will work for Krells, but ask them first just to make sure.