Pondering REL and Dynaudio subwoofers.

You're considering two very different animals. 

If I'm understanding Dynaudio Sub6 spec "(Hz @ 85dB +/- 3dB): 16" as well as the manuals suggestion of placing the unit at the listening position to excite your rooms unique standing wave bass modes to position the Sub in, ie: the subwoofer crawl, the Sub6 is a true -3dB@16Hz subwoofer. Aside from this more detailed room integration the Sub6 appears to offer an application for far greater seamless speaker integration. In my experience with a big REL you ain't heard nothin' yet.

All REL products are merely Sub-Bass Systems (woofers) with a dramatic -6dB roll off that simply does not play loud enough to excite a rooms standing bass waves. This allows them to be placed most anywhere in the room. If your not concerned with hearing the delicate (not boomy) extra low frequency presentation found on most recordings and do not want to be bothered with locating your rooms standing wave modes then the Classic should do just fine, minus the Sub6's full presentation. 

Your amplifier manufacturer is the ultimate authority on grounding resolution not REL.

Your Freya has three sets of outputs. If the Sub6 position is a long distance away from the Freya you have the option to use inexpensive custom length XLR or single ended cables from Blue Jeans or Monoprice Cables. 

All the best with your search.

@grantgg 

d issue of a hum when my class D  Orchard Audio Starkrimson Ultra is powered off (also use Freya+ F)

it is a ground loop problem of sorts known to happen with class D amp

This sounds like a two part question.

1. If all your devices are 3-prong then the solution is to use a ground loop isolator, coupled with manually grounding the devices together, mentioned above. If the amp is off and there's a hum, then it seems to me the problem is another device that is powered on. The classic 99 or Sub 6 both use class D amplifiers and are 3-prong. The subs can also be the source of the problem.

If you're mixing a 2-prong devices to a 3-prong amplifier, it can cause a ground loop hum because the difference in grounding methods creates a path for noise. The 3-prong amp is grounded to the wall, while the 2-prong device is not, creating a potential difference. With this issue, you still need a ground loop isolator. A different subwoofer does not resolve the problem. Also, subwoofers often use class D amplifiers with a switch mode power supply and the switching frequency contributes to the hum or high pitch whine.

2. As to how the subs compare, per a REL video I've seen, the classic 99 is designed to give a slower, smooth bass that is a good match for vintage speakers or if you like that sound signature. Just looking at the Sub 6, it's designed for more punch and speed given the sealed, dual opposed drivers.

@bartsw  Yes I have one more fix to attempt, I've tried several others, which is putting a three-prong to two-prong a/c adapter on the subwoofer and or amplifier, that had been advised to me by someone.  I've yet to try that.  The Rel uses high-level connection which is where the hum issue comes into play.  By using the low level connection, hum is not an issue, but I think the Rel performs best on high level in my testing.

  But I am torn between keeping the classic 99 or trading it in for the Sub 6.  As @m-db  has detailed so well. There are big advantages with the sub 6 that really appeal to me and my tastes. I would position it between my preamp and amplifier for its full DSP capabilities.  Only downside is that I have to get some more XLR cables to connect them all.  My Starkrimson only has XLR inputs.

Since this is likely my last subwoofer I'll get, i should choose carefully.   

@grantgg 

Yes I have one more fix to attempt, I’ve tried several others, which is putting a three-prong to two-prong a/c adapter on the subwoofer and or amplifier

Somewhere I heard this is not safe.

The Rel uses high-level connection which is where the hum issue comes into play.

OK. I also have this problem with high level, not low. You might be able to resolve this without a ground loop / hum eliminator. I use an SVS speaker level subwoofer adapter since my sub does not have high level. Try this:

1. connect the positive line from the class D amp or speaker to the positive on the adapter (you directly to the sub)

2. leave both left and right negative terminal disconnected on the adapter (for you being the sub) from the class D amp or speaker

3. connect a line from the ground screw / or chassis screw on the class D amp to one negative terminal on the adapter (you directly to the sub)

I would check with REL to see if this is OK. They might have a video on this. They will ask if the amp is differential, which it likely will not.

REL tutorial video mentioned above but the instruction is slightly different:

https://www.youtube.com/watch?v=NFbcAgpBID0