I don't know how changing a Tri-Planar / Teres-320 to run a Stelvio armboard would affect to sound. With the Serac's MDF base, the benefits of the Stelvio armboard are someowhat masked. I suspect the hardwood Teres will fall nearer to the sonics of the Gavia/Stelvio, but I can't say for sure.
2. Mounting issues.
a) You'll want the surface of the armboard to be about 1-3/8" below the platter surface. The height of the Stelvio armboard and the stainless spacer comes in at a touch less than 1.500", so some simple subtraction will tell you what you need to know.
The Tri-Planar has quite a range of adjustment in this regard. Between this and the fact that spacers like those used with the standard armboard can be employed to raise the armboard, I doubt you'd have a problem.
b) Lining everything up. With it's offset mounting center, the Tri-Planar takes a bit of playing around to get the right pivot to spindle distance while retaining an ergonomic orientation of the arm when it's parked in the arm rest. You've probably seen posts of mine on Audiogon describing that one should not depend on the Tri-Planar mounting jig.
Drilling the armboard without having the turntable in front of me can be done, and it would require that I know the following:
- the distance between the record spindle and the armboard's pivot bolt.
- the diameter of the tt base at its widest point. Hopefully it will be 13" or less. If it's much more than 13" the Stelivo damping cylinder might interfere with getting the correct pivot to spindle distance. Slightly more than 13" can likely be compensated for during mounting.
- the radius of the arm mounting area of turntable - the narrow part of the cam-shaped base.
With these three pieces of information, I can mock up the base dimensions and do a mounting deserving of the Tri-Planar and your turntable.
c) The fact that the Teres arm mounts with the screw feeding from the under side of the turntable as opposed to my method where the bolt feeds from the top (into a helicoil in the turntable base).
Reversing the orientation of the mounting bolt, requires that a 3/8 x 16TPI nut (or alternatively a T-nut) be located on the under side of the base in order to thread the bolt into. For the MDF Serac base, I bore a 1.25" diameter hole on the under side of the base, and fit a T-nut through this hole. Only the top 3/4" has the diameter of the T-nut's shaft, with the large counterbore providing access to insert the T-nut. Obviously, doing this to your hardwood Teres is an irreversible mod and I'm hesitant to recommend this sort of thing unless someone is sure they're going to keep their turntable.