Before you pull out you test gear, try a couple of low tech (read cheap) things first. Since it's a new machine (for you) do a thorough cleaning of the heads, tape path, rollers, and connections. Sometimes a bad RCA connection can do exactly what you're describing. Also, poor head to tape contact can make dynamic peaks fuzzy. After cleaning the heads/tape path (pay particular attention to the outside edge, away from the deck plate) examine all the points of contact with a decent light and a magnifier. Sometimes a clump of really tenacious gunk will hide behind the head shield. If that doesn't help, try putting a finger on the supply reel as it plays to increase the back tension. If the volume increases, your tension probably needs to be adjusted. It could also be caused by misaligned heads but if the machine hasn't been played in a while, whatever the tension mechanism is may be gunked up or worn (springs stretched?).
If your machine has an output level pot, try moving that around. It could be that it's full of dust and gunk. If it sounds scratchy, get yourself some DeOxit D5 spray, find the back of that pot, and squirt a little of the D5 in any opening and work that pot back and forth 20 times. If it seems to get tight, stop and try it again the next day (forcing it can ruin some pots) when the DeOxit is totally dry.
All these things need to be done periodically to any machine. The longer it's been sitting unused, the more likely that things are gummed up. Chris had some excellent advice and would know better than I would on the technical end of things. Fortunately/unfortunately I'm pretty experienced with gummed up machines.
BTW Welcome to the forum.
