ARC Vsi60 with KT120 valves - issues

Hi guys,
I'm a newbie here and also new to the world of tube amps. I bought my Vsi60 back in July and it came supplied with the KT120 valves. I did what I considered an extensive demo with my speakers (ProAc 1sc) and my CD player (Cyrus CD6SE) at the shop and was blown away at first. However after receiving my amp and setting it up at home, I felt that the sound was way too bright. Violins sounded thin and edgy, and the overall sound was one of brightness and artificiality (violins in some recordings sounded like synthesizers to my ears!). At first I thought it was a burning in process but after about 150 hours or so, I decided I had to do something about the amp or sell it! So I decided to get a quartet of Winged C 6550 and changed the preamp tubes to the EH 6H30 (with the gold pins) and the sound was exactly as I hoped an amp would sound. It was rich, lush - instruments and voices sounded very lifelike with just the right amount of edge to the tone to make it sound clear without being harsh or artificial. I love the sound and my amp now!

However, I am still curious about the KT120 valves since many people on this forum have expressed their preference for this valve. My query to the more experienced guys out here is - what is the correct bias for the KT120 in this particular amp? The booklet that came with it only states the correct setting for the 6550 valves (I kept the same setting for the KT120s) and the shop basically told me to stick to the same settings for both tubes.

Since ARC does not have a customer email address and I live in Australia, I would appreciate some feedback regarding making this KT120 sound at its potential best particularly with regards to bias settings etc. As it is my first valve amp, I am afraid of blowing it up or taxing it if I play around with the bias settings without understanding it more.

Thanks in advance.