IMHO, the Acurus is the weak link, and though not because of that, the least tube like sounding component in your system. It would seem that the Acurus would be the component to consider replacing with a tube unit. But a bit of caution might be in order. I can't seem to find the specs on the DNA .05, but different models of McCormack amps had various input impedance's and sensitivity. Some had input impedance's that might be a bit low for some high output impedance tube pres, and others had a rather high sensitivity for some of the tube pres that have higher output, compromising the usability of the pre volume pot. Using a tube pre can work, and work well, but as always do your homework first.
Matching up speakers and amps
I currently own a pair of Spendor A5's that I am very happy with. They are matched up to a McCormack DNA 0.5 and the Acurus LS11. I am looking to replace one or both and also still debating tubes vs SS. Any opinions as to the Spendors matching up better one way or the other?