If it takes 340's to spine up right with 100 gn tips, then 400's would definitely be way underspined with 125's.

You may know this, dwd, but there have been several questions about Easton/Beman sizing here lately, and I don't know if it's ever been fully explained. So, hang in there with me for a minute.

Easton and Beman size their shafts by deflection (amount of bend) in thousandths of an inch, using a spine tester. Deflection for the 340's is .340", and .400" for the 400's. The further the shaft bends under the weight, the less stiff the shaft is. So, size 300 is stiffer than a 340, and 340 is stiffer than a 400.