The masters will all be either Dolby A or dbx1. No one ever mastered in Dolby B. The process was this, the master was dubbed and decoded, and then copied to the production high speed master, and this is where the Dolby B code was added. However, no one copied in real time. So this was the high speed duplication version. In high speed duplication there is always HF loss. This required a boost of HF to the duplication master. So if someone did make copies for sale from a duplication master, even if they used Dolby B decode, it would be HF heavy and not sound right. I can not imagine some idiot doing that, but anything is possible.
My best guess is that they used the Dolby A or dbx 1 master and did not get the levels correct if it was Dolby A, and there were frequency response errors between the master recorder and the recorder used for making the production dub if the master was dbx1.
As I think about this, it is very unlikely that the problem you are having is a Dolby B decode problem. It is almost certainly one of the issues I have raised.
I used to make professional recordings back in the day, mainly for radio broadcast, but I did have a high speed duplication system to make Dolby B tapes. So I know what the process is, and what great care you have to take to get it right. All machines must be in top condition, and you have to really listen to the high speed copies and remaster the duplication tape until you get the HF balance right. Most did not, and so there are a lot of less then optimal results around.
So I think this notion, widespread on the Net, that this is an issue due to not decoding Dolby B code is almost certainly NOT true, and much more complicated.
You have to understand what the process is, to understand what has likely gone wrong.
There is an Advent 100 A that has gone on eBay since last night.
One other caution, even if the tapes are Dolby B encoded, unless you have the level tones on the tapes, you will have a huge problem getting them to sound right. That is what those level meters on that Advent unit are all about. The Advent unit is the best stand alone Dolby unit, and is engineered correctly.
I have some commercial Dolby B reel to reel tapes and they are headed by the tones.
The only commercial real time tape duplicates that I know of where from Everest, and they are on reel to reel half track stereo 7.5 ips. I have a few of those also.