Try calling Synergy Audio and Visual, they are the Australian importers for ARC.
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When I got my Octave V110 (designed for the KT120's) I loved the sound for about the first 30 hrs (a very warm engaging sound). After that they sounded too forward in the mids and the highs were a hair rough. After 200 hrs they seemed to settle down. Bass is nice and punchy but the mids seemed a little too detailed but the highs smoothed out. Changing the drivers (12at7's) to Mullard's (very warm) helped a lot and made them sound acceptable to me.
That being I am somewhat spoiled. I prefer the Ei KT90 over the KT120. My next favorite is the vintage TS 6550 solid grey plate with my absolute favorite being the vintage TS 6550 sold black plate. Now I will admit the only tubes I can run at 110W power (620V plate voltage) is the Ei KT90 and the KT120. The others I can only run at 70W power (540V plate voltage). I do like the added 'control' of sound at 110W. I also have SED 6550 winged C's and SED KT88 winged C's along with the Gold Lion KT88 re-issue. The original Gec KT88 is very nice but a bit dry sounding like the SED 6550 and KT88's. Vintage Tesla KT88's are very nice but like the EAT KT88's they don't like the 540V plate voltage. Lost 3 of 6 EAT kt88's and 1 Tesla KT88 in less than 30 hrs.
Anyhow that's my opinion and the input and driver tubes can make the KT120's sound better but you have to try them for yourself to get the sound you like the most.
Although I understand that mentioning tube rolling results in another brand of amp might be meaningless, I offer the following: I've used the stock Tung Sol 6550s, Sovtek KT88s, and KT120s in my Jolida 502p. All sounded great, but I prefer the 120s (driven at the amp's recommended 500mv bias for no particular reason except maybe the stock specs, and the sound) as they seem to have a really good grip on things without the harshness or lack of soundstage noted by others. Also noted by others is an overwhelming acceptance of 120s generally, with criticism being surprisingly rare (thus welcome as I get bored with too many positive reviews of anything). I read a review of a newish ARC 75 watt integrated recently and it was found to have harsh highs by that reviewer, which is surprising and simply TAWDRY for a ARC amp costing around 10 grand. I've had some success in making a good thing better with driver tube swapping...currently using Psvane 12AX7-T IIs and Mullard CV4024s (12AT7) that seem to give the 120 tubes (and my ears) room to thrive.