Well, I found this ebay listing:
http://cgi.ebay.com/Nakamichi-AV-2-Home-Theater-Receiver-Dolby-Pro-Logic-/140406713933#ht_5311wt_1165
With some good shots of back of AV-2
Not sure if you know it, but AV-2 is Dolby Pro Logic only receiver, which means the only way you get "5.1" out of it is by applying DPL logic to
STEREO input signal, which is a far cry from real 5.1 surround.
Which inputs your htib sub has? Or even better post a picture of it's back...
Keep in mind this model is not real Nak, they were bought and sold several times re-using the brand name to sell their own products.
About this time I believe Grundig was the owner.