Measuring resistance in circuit is a bit tricky. The reading will be affected by all possible electrical paths possible i.e. other resistors and even semiconductors (if there is sufficient current to turn them on). Even pro electronic techs lift one end of the resistor for accurate measurements. It is possible with experience to check resistors based on the voltage across them when the power is on. The good news is that resistors usually are either good or when bad are an open circuit (burned out) they rarely change resistance or cause problems like intermittent faults (old carbon comps can shift value but they are not used much except by guitar amp and FX manufacturers). In my many years of repair the most common fault I found is bad connections (cold solder joints, bad grounds, dirty connectors, bad tube sockets etc.) followed by bad semiconductors. But back to measuring resistance I do voltage readings around the circuit while it is operating and find the bad component by analyzing the various voltage drops. Again experience is the most necessary tool in the toolbox. Of course sometimes it is not possible to just run the circuit if it is drawing excessive current but there are some tricks that I use to get around that problem. The best thing to do is keep trying, study schematics and most of all studying electronics. Sorry for the long answer.
