Hi,
I've had the VTL 5.5 and currently own the Premier 10.
both drove a CJ 11a and SF Concerto, with Cal Alpha as source. I had them one after another but not together at the same time to really compare them properly.
They had a lot of similarities tonally in the mid and upper range, but the VTL by far had the more fuller, robust base. I think the VTL had a more laid back midrange specially on vocals. This may be the reason why, for me anyway, I found it slightly unengaging or uninvolving. The Prem 10 is also a bit more open and transparent and although I found the base borderline acceptable, it is for me,as a whole, more involving than the VTL.
I'm also curious how these two preamps would compare with the Cary SLP 94 or 98.
Hope this helps.
