ARC VM 220 WITH VANDERSTEEN WOOD QUATRO

I am looking for a little guidance, The pre is ARC LS 25mk2 and the amp is ARC VM-220 mono's hook-up to a set of Wood Quatro's. When i turn up the volume past half way, I seam to be losing the warmth of the tubes, Its sounds alittle (forced, less detailed) not sure how to explain it, just different. I biased the tubes all at 65. was wondering if i need to switch to Tung-sol KT120 tubes or go with a different set of speakers, higher in sensitivity, or is this headed in the wrong direction.thanks