The only thing I know about Amvona in general is that they sell a lot of stuff on Ebay, and people who left negative feedback concerning shoddy parts/customer service got neg. feedback back. The only product of theirs that I've seen first-hand were large scrim panels, and they looked like they were decent quality for what the guy had paid for them.
I have tried their ball heads and they are definitely sub-par.... stick with manfrotto/bogen, gitzo and other bigger names... it's worth every penny.
i bought of couple of amvona ball heads for less than $100 and they seem to work fine in the beginning.... but they start falling apart. Things were slipping... one time, I my D2X nearly felll off. I had to just throw mine away after less than 6 months. contacting them is a pain the the neck also.....
i ended up spending $150 on a manfrotto/bogen ball head.... never looked back and having a ball of a time with it. Works as it should and solid as a rock!
I have an Amvona tripod head. It is an Arca Swiss knock-off, with emphasis on knock-off. I will say that it is worth the $60 I paid for it, but it was meant to be a temporary solution. I spent more than I planned to on my tripod and needed a head, any head, in a hurry. It will be replaced by a Markins or RRS as soon as I have the funds available. It does not hold as true as a head should.
It may look like the more expensive heads it's trying to emulate, but it is NOT of the same quality. If I had to do it over again, I would have bought a Flashpoint head from Adorama as my temporary solution.
PS- Still loving the Tamron 17-50, by the way . . .