Gain Matching - need advice

By using xlr interconnects your db output is 2 x as much ,the volume pot on the Joule also seems to be much less , 
with speakers this efficient  did the dealer give you a 30day audition  for comparability ?  You should  try a pr of rca interconnects , just to see how much gain and if thisis then suitable , if not the preamp  is not a good match and needs to go 
back ,or sell it ,  Don Sachs would build your custom preamp 
to have ample gain , for I doubt the AR volume control is a shunt, resistive ladder type, Aron Sachs uses a A Khozmo which I have used several times , if you want a true top of the Line RaduTarta
builds DHT custom preamps using all top transformers and 
can build to your amplifiers output for $5k you can have a preamp 
with far superior parts quality.I know for not only do I know about 
modding or upgrades for years ,but he is a 40 year expert in vacuum tube equipment SET amplifiers , and DHT preamps are by far the purest but. Incredible build quality and musicality ,even the exterior can be custom wood or metal and he uses several excellent brand  volume attenuators , which can make or break 
your preamp , for your speakers you have Waay too much gain 
in all honesty you-should have agreat 8 watt set amp , your speakers use only 3-4 watts ,the gain is high ,any speaker with over 100 db efficiency  , made for a high quality low power amp .
SSamps typically have too much gain send Nelson Pass a Email
See what he would do , his great 20 watt amps for around $4k
woould be ideal ,not 60wpc . Send Nelson an email he will help you  things out . If you want a great vacuum tube preamp,or SET
tube amp Radu Tarta without  any hesitation ,and per $dollar spent 
by far the best parts quality ,for he builds each one by hand 
and no dealer markup . You discuss what your budget max is 
and then take it from there , do a search ,build quality for a life time at very fair prices.

I had a similar problem. BHK Sig. pre > Jeff Rowland Monos > Spatial X-5's (97dB, 8ohms). The volume level was low but manageable, but I was having a lot of trouble with tube noise due to the gain of the preamp. Someone on the PS Audio forum suggested Rothwell balanced -10 dB attenuators and it worked like a champ. These go in the amps with the IC's into them. Very little noise and the volume level is now around 30 for casual listening, 50-60 for blowing the doors off. Rothwell also make -20dB attenuators as well if you need to go that far. Well made, reasonably priced, and no issues for me. 

FWIW I agree with markmuse. They are cheap and effective. Obviously they require another set of cables and you (as some others) might find them less than transparent). If you can they would be great in a tape loop. If you found you really liked them you could consider moving on to a mod in your pre-amp to do the same thing. I'm not familiar with the ARC 6 but in other ARC products it has been a simple and reversible change. You can always call and ask. 

You need to throw some gain away, obvious is to dump the preamp/s. And use a 10kohm passive pre so anything can drive it. (save for some silly high output impedance sources) (usually tube).

Forget inline attenuators, too much of a gamble, you never know if you have an impedance match or not, (unless you know how to measure them) because from what I was told they don’t state what series/shunt resistances they have used inside them

Cheers George

You do have a gain problem because of the having to use balanced interconnects it gives you 12 decibels more gain than what you think 6 db from the pre amp and amp each that would give you 32 plus 18 which is 50 db of gain way too much on an avantgarde if you could hook it up single ended you would have 38 db of gain ant that would be ok on the avangardes i would think but maybe still a little too much but nothing like the 50 db you have now.