Zen amp: Worth upgrading to Select version?

I just discovered the Zen amp magic after pairing with the AES AE-3 preamp last night. It sounds wonderful on my Axiom M22Ti.
Frankly, I can't really tell if my 300B SET sounds better or not (need more critical listening).

Anyway I wonder if it's worth $150 to upgrade from SE84C to SE84C-S (Select) version? I mean do the $150 pay off for the upgrade in term of sound quality?

I would really appreciate if anyone who use both can give me some comparison thought on them.

Thanks,

Ake