I've spent the past few days listening to the SIT-5 and XA60.8 back to back (using Nordost Tyr 2 RCA on both to eliminate that factor). In retrospect I may have been a bit harsh on the 60.8. While it's definitely missing the greater holographic imaging and texture of the SIT, it's not really grainy and dull - just less good especially in the highs and mids.
I cannot speak to synergies with your Tannoys and it may depend on your music preferences. I suspect Nelson may offer an opinion as he's familiar with them.
As a side note, a friend recently asked how the SIT does when really pushed with challenging music. To those surprised that they mate well with Wilson Alexias, while I listen mostly to classic jazz and pop up to moderately loud volume levels, to answer my friend I played the Scorpion's "Still Loving You" full tilt. The vocals demand everything from the highs and mids, and when the percussion/bass come in the lower end is comparably challenged. All I can say is that at ear-splitting volume it put a smirk on my face - gives very little away to the 60.8 in bass slam, etc.
As Nelson says, there's no perfect amp, so along that philosophy if I were primarily listening to rock and/or other challenging music at high volume, I'd go with the 60.8, but for jazz/instrumentals/vocals it would be the SIT - no question. The SIT will play loud with exceptional performance in the upper range, giving away something in shear dynamics/overhead but it's magical with richer texture, detail, holographic imaging, etc.
