I haven't tried Stan's 29er rims but I can speak from first hand experience
that the Alpha 340s are NOT up for cross. I had a rear built by Wheelbuilder.com
and it didn't last a season w/o a sizable dent in the sidewall. This season it is now bent
between three spokes and is impossible to keep true. After this season (it's 2nd) it will be toast
and the hub will have to be rebuilt into another rim.
OTH, I've run 26" Stan's 355 mtn rims in all types of racing and training conditions and they have
required only minor truing.
I'm running the Alpha 400s with Specialized Captain CX 34s (tubeless). I ride pretty aggressively and I live in a rocky area, but haven't raced them. They are holding up really well.
As a side note, I ordered the wheels with Stan's Raven tire because I wanted something straightforward to mount (I was nearly homicidal after trying to get Ksyriums tubeless). Ironically, the Ravens were unusable as they were too loose so I sent them back for a refund (I couldn't do it and two shops couldn't either, so I gave up).
as an aside 2 i ran kysriums es with hutchinson piranhas.
a bitch to get on, nearly impossible. to get off i had to cut the bead. but they sealed right up.
they didnt have enough traction and were squirrely, but i ran them at 35 without issue. but i am lighter and
That's what I attempted. After struggling to get them on (and I really mean struggling), I got the rear to hold air, but the front bead exploded off the rim as I was trying to seat it with compressor. Nearly lost my hearing
I'm using Crest 29er rims and American Classic hubs as my race wheels. I've used them for a couple of seasons. I really like the set up. The wide MTB rim really allows you to run lower pressures with the tubless tire. I use the Raven for dry conditions and Michelin Mud 2's for in the wet. The wide rim also seems to provide more breaking power and the anodization on the rim makes the braking nice and 'grippy' (until the ano wears off).
All in all I'm happy with the set up.