Iain: Just looked over alot of your past posts........some thoughts I have.
Your running basically an F12. Not sure what size room your in, but your already at a disadvantage compared to F/D15 users in that your amp basically needs to double up the output power to have the same output level as the 15" users. Again, I use a PAIR of D15's in a modest size room. Even though you state you don't listen to music at "reference levels"(Which I believe is in the 105-110db range technically), you most likely pushing that F12 pretty darn hard. I actually can believe that you do in fact notice a difference when it is on. IMO based off this, you probably should have opted for an F15 or greater even, especially if your room size is larger than where I have my pair of D15's situated. Im actually going to be picking up another pair of D15's when the new XLR 2/3 amp is available. THe lower the gains, the better. In your case you should probably think of picking up another 12" Rythmik, I think an extra 6db output and clean headroom will serve you well.
Im actually surprised you didnt go with a 15" model originally. The only time I actually recommend the 12" Rythmik to people, is if space, total cost and listening levels being LOW under normal use is the main factor. I have found regardless of the size and material of the Rythmik driver used(GR paper cone, or regular aluminum), all the Rythmik subs blend in equally well to small monitors well above 120hz as the xover point. Im a big audiophile myself and a long time bass player, and find the even the D15's with a downfiring 15" driver can blend in seamlessly well above 120hz with small monitors that feature a 4" or 5" midrange driver. That is how good these subs are. The reason I feel the Rythmiks are absolutely the best audiophile subs out there right now at a reasonable cost.
WIth that said, one thing most customers don't do or fail to realize though is to properly match up the subs true clean output capability with the equipment they already have along with the size of the room and their normal output volumes. Most people I find significantly overdrive their sub system beyond what its capable of doing in these terms. Just like your main speakers, subs for music and movies need to be able to and will produce dynamic swings that are greater than 10-20db. The F12 has about 27-28db of clean headroom based off 1 watt RMS. So even when your listening to music below "reference levels", your sub and mains still probably are put to the task for quick intervals. Your subs sensitivity db/watt naturally is probably in the low 80 range in your room at best and probably at LEAST 6 db down from your mains at 1 watt of power if not more. This means that when your main speakers have 1 watt rms going through them, your subs have at LEAST 4-8 watts RMS at the same time output. So there is without question the good possibility that your single F12 has less than 10-15db of clean headroom in your situation and is probably why you notice SQ differences with the limiter on. You are most likely hitting quick transients that require more output than what the plate amp can give. I bet if you add a second F12(Always a GOOD thing
)and gain another 6db of headroom, you would notice the limiter to be less of a factor or a non factor period. And in the process put alot less wear and tear on the plate amp and voicecoil of the sub as well in the process. And since you listen to mainly music IMO you should have a pair of subs regardless and have one per monitor. Not only will you have less potential localization, but your soundstaging will improve and the rooms bass response will naturally smooth out as well. Rythmik subs run best in PAIRS.
To add, with one of my systems that feature a pair of small modded Kef XQ10's(with an in room sensitivity in the mid to high 80 range) and I need a PAIR of D15's per SIDE(Currently I have one D15SE per side) at 20-40hz to technically equal the sensitivity and clean headroom of ONE XQ10 at 1khz that has about 2-300 watts of clean RMS/dynamic power(And this would basically be with any 15" sub, not just Rythmik)going through it. People who hear my system cannot believe how effortless the sound is on my system with such small bookshelf monitors. Its due in part because the sub system is matched up near correctly in terms of power and dynamic headroom to the main monitors and amplification. And I actually feel that I need at least one more D15 in this setup to be 100% optimal in terms of gain/power matching if not 2.