I've auditioned the 3A Sigs. with Quicksilver mono-blocks (similar to the ones Richard Vandersteen uses at home), Theta's original Dreadnought and the BEL 1001 Mk IV and Mk V. So far, my preference is for a pair of BEL Mk Vs. A single BEL is next up, then the Quicksilvers, last the Theta. The BEL is the most convincing and it drives the 3s well whether it's in stereo or mono mode. Mono-blocks are the best as they give the best sense of presence and size amongst other things. The Quicksilvers give a little of the tube bloom without giving up the frequency extremes. They however, do give up the detail that the BELs have. The Theta is now out of production, but it was not as good as the BEL Mk IV in stereo mode...not by a long shot. The BEL Mk IV smoked it. I suspect other amps such as the Spectron might sound good as well. I didn't A/B the BEL with the Spectron on the 3s though... On the 5s the BELs won hands down.
In the end, it's personal preference. The Vandersteens have the ability to show exactly what's upstream of them. So system tuning and matching becomes important if you want the best sound from them.