Need help from Pass X-600 + ARC Ref 2 MK1 user

I am matching Pass X-600 with ARC Ref 2 (MK1) and encountering a strange hum issue. I have tried couple combinations and it seems like the hum is coming from only X-600 + Ref 2. X-600 and Ref 2 do not seem to have any problem individually. Following are the combinations that I have tried and they show no hum at all.

1. X-600 + Pass Aleph P
2. X-600 + Wadia 861 (using build-in preamp)
3. Mark Levison 331 + Ref 2

I have also tried couple power conditioners and even power regenerator, but the hum still. I was told the hum might be from power & preamp impedance mismatched. However, it shows pretty healthy from the specifications. Any comments on this will be appreciated. Thanks in advance!
Dumb question time. Have you tried using a cheater plug on the amp?
Newbee: Thanks for your comment. Yes, cheater plug is the very first thing that I tried. I think I can pretty much rule out the ground loop issue.
Does it humm out the speakers or is it a mechanical hum from the transformer of the amps?

Are you using balanced cables?
Well, here's another dumb observation....If you are listening to noise with your ear near the tweeter/midrange and if your amp has a highish imput sensitivity you might just be listening to the noise floor of your amp, or its tubes. When I crank up my SP10II I get both a light hum and tube rush using two different amps, one with a .5 and the other a 1.v input sensitivity. With a third amp and a 1.5v sensitivity I get very little noise or hum. A different tube pre-amp is dead quiet into the same amps. Both with my ear at the speaker.
Cytocycle: It humms out of the speakers and a little bit from preamp's transformers as well (no mechanical hum at all on X-600's). I have tried PS-Audio's HumBuster, but it doesn't help on the mechanical hum from Ref2. I am using balanced cables. X-600's take only XLR.

Newbee: Humm... I can't find input sensitivity from my X-600's spec. Do you imply that there is some kind of mismatch between X-600 and Ref2? The humm is somehow auditable from the listending position.

Nope. Not implying anything. I'm sure your X600 is probably just fine and the match up may be fine as well, just that the noise could be normal. I thought you might just be new to tube pre-amps. Continuing on that assumption....

Tube pre-amps in general just are not as quiet as SS pre-amps. Do you hear the hum when the volume control is at its minimum level or do you only hear it after you rotate the contol well past 12 o'clock? Do you hear the noise with any source input selected or only with phono and with a phono pre connected?

Hearing the hum at the listening position is not good if your volume control is any where near a normal listening level or at its minimum. If you are hearing this hum with only certain sources, like phono and only then with the volume control rotated well past a normal listening volume you may have a poor source or, in the case of phono inadequate gain for your cartridge. With some some sensitive amp and efficient speaker combos amps I have inserted in-line attenuators to bring the "noise" floor down to inaudible levels. Am I helping you yet?
If it humms out the speakers it's a ground issue, I tried a hummbuster also and it didn't work for me so it's not a huge DC problem. The Pass's already have a DC filter built in.

Have you tried the preamp at someone else's house to see if the transformer humm problem or an interaction/power/ground problem. ARC has great support.

Try floating the ground on the CDP, and then preamp , and then break the ground on one of the amplifier monoblocks (not both).
I had an identical Humm problem on ARC LS2B into a pair of Rowland 7M monoblocks. Humm was from speakers and was insensitive to volume control. Humm went away when I applied a ground cheater on LS2B.
I have tried Mark Levison 331 + Ref 2 at friend's house and no hum at all. The humm doesn't increase as I increase volume control. The humm occurs without connecting any sources from preamp, simply power amp + preamp. I have tried star connection and leave only one ground for fixing ground loop, but the humm still.

Newbee: What's in-line attenuators? Where can I get one? Will it affect/downgrade the sounding?

Thanks for all your comments.
The in-line attenuator that I was referring to is made by Rothwell. It is, in essesence, a resisor that is placed within an adaptor that you plug into your amp and then plug your I/C into it. You can buy these on line. They reduce the pre-amp output by 10db and in my system its enuf to lower the pre-amp's line stage noise floor below audibility. Cost about $60 on line for a pair.

About your problem - I've not a clue. But since it wasn't a problem at your friends house, it must be something in your power supply. Do you have a dedicated line, etc?
i have an arc ls25 mk2 with bryston 7bst. these components i believe have very similar impedance specifications as ur arc and pass. both components are dead quiet and even with my ear to the speaker, no noise.

i don't think its an impedance mismatch. i recall reading something from the pass website that the x600 amps can be driven by anything and impedance mis-matches is rarely a problem.
No, I don't have a dedicated line. I thought about the power issue at my home before, but why even the power re-generator (PS Audio Power Plant) still doesn't help?

Another symptom is that I still can hear the music playing while turning the preamp volume control down to the minimum. Therefore to reduce my preamp (gain) output can probably benefit to both... I will do the research on in-line attenuator. Thanks for your info.
Sounds like an internal ground-loop. I fix lots of these in my mods. They are common even in high-end gear.
Internal ground loop? If that's the case, which unit needs to be fixed? Power amp or preamp? Thanks.
After reducing the output gain from preamp, the humm goes away! Thanks for all your comments!