The temperature is controlled by the thermostats. The pressure relief valve sets the maximum pressure that can be developed in the boiler, and thus in the entire brew path. A change in OPV setting will have little effect on brew temperature in that machine. If the taste is harsh, biting, sort of "rude," and as tekmino said, sharp, then lower then pressure a little. You may have to do that a few times to find the best setting. Another guideline is that if you have to grind finer with the machine that is delivering poor espresso, then the pressure is too high.

I keep forgetting that single boiler machines don't have p-stats. I've been an HX guy for so long, you see.

Still, the problem might be brew pressure related or might not.

In any case, it's always a good idea to check on brew temps, because it's such an easy, and consequence free thing to check. For another, boiler pressure and brew temperature are interrelated. PV = nRT and all that.

If, as I infer, the brew from the problematic machine is too bitter, your temp is probably too high. Instead of waiting 30 seconds after the light's gone out try 1 min, 15 sec; then try 2 minutes. See if a cooler temp doesn't get you closer to the sweet spot. If not, you can adjust the pressure with nothing lost but a little time.

BDL