The safest, easiest way to check grounded outlets is a little checker you can get at the hardware store or home center electrical department. It looks like a grounded plug with some indicator lights on the end, and depending on the combination of lights lit and extinguished when you plug it into the outlet, it can tell you if the outlet is wired correctly and functioning and what the mis-wire is if it is not. Just a few dollars, totally safe and very useful when you have doubts about an outlet.
Of course, you can use your meter on the appropriate VAC scale to check an outlet, though sticking your meter probes into a live outlet requires great attention and care to avoid a visit from the bright blue Jeebus! You will notice that, of the two flat slots, one is wider than the other. The wider one is supposed to be neutral, the narrower one "HOT". The round hole is safety ground. So, if you measure between the two slots, you should read line voltage of approximately 120 VAC. Measuring between the wider slot and the rounded one (Neutral to Ground) should give you virtually nothing. Measuring between the narrower slot and the rounded one (HOT to Ground) should give you 120 VAC again.
I cannot stress enough that you should only try this if you feel comfortable that your meter probes will fit comfortably into the slots on the outlet, and that you can take the measurements without running the risk of the probes ever touching each other! A pair of leather gloves would be a nice safety item, as well as shoes and sox!
Or, just spring for the outlet checker and avoid the worry.
Another, perhaps easier, way to do this would be to just plug the amp in and check this at the power inlet on the amp. You will notice on page 16 of the manual in the upper left hand corner of the picture the terminals are marked "L" for "Line" (HOT), "N" for "Neutral" and "E" for "Earth" (Ground). If the amp is plugged into a correctly wired grounded outlet, there should be approximately 120 VAC from "L" to "N", virtually nothing between "N" and "E", and approximately 120 VAC from "L" to "E".
If that checks out, then you should get correct readings at transformer terminals 1 and 2. As Paul points out, the readings you get at some of the other transformer terminals can be dicey depending on how well your particular meter discriminates between AC and DC voltages on the same node. But, if you get things squared away on transformer terminals 1 & 2, it will indicate that you have a good safety ground, and your earlier checks seem to confirm that you've got a good circuit ground, too. That is important, both for safely operating something with a metal chassis, and for the circuit to behave properly.
After that, the DC Voltages will tell you if the amp is operating within normal parameters.
If in doubt about any readings, just post them as you did before, and I'm sure someone will chip in, either with an explanation or something else for you to check!
