I don't really see a point in buying a 28mm when you have the Tamron at f/2.8 already. Sure the prime would have a slightly better IQ, but not enough to justify buying it (imo).
As for 50mm's, I already told you. See my above post.
You could also look into the Helios 44 (58, f/2) or SMC Takumar 50mm f/2 (not really familiar with this lens, but apparently it's good). Also check out the Olympus Zuiko 50mm f/1.8, pretty nice lens for the price (25-30$).
No experience with Volna glass, never even heard about it. It's probably some rebranded glass made by another company. Your best bet is to check out MFLenses.com and do a search there.