Audio Modeling have just dropped the beta of the iOS version of their String instruments, following the rollout of their Brass instruments earlier this year. (I still have 32 days left on the original betas for those…) As with the Brass beta, anyone can sign up, and you get one each of their Violin, Viola, Cello, and Double Bass instruments for 88 days. A quirk this time around is that you need to tick the box saying send me marketing e-mails before they'll actually send you the Test Flight e-mail, but they haven't spammed me so far.
These are slightly less compelling than the Brass because there are already a cut-down but very playable SWAM Violin and Cello available as IAPs in GeoShred, which periodically go on sale at $9.99 or half the regular price of each of the standalone/AUv3 instruments, and have GeoShred's pitch rounding as an additional attraction for some. But if you're at all tempted by the new instruments, these betas are fantastic for helping you make up your mind, whether or not you find anything useful in the testing.