The only tool i use is my ears.
Chris, you can't rely on your ears to ensure that your speakers are outputing the same SPL. If you don't ensure that they are at the same SPL, the comparison will not be as valid. Tests have proven that the loudest speaker usually wins even if that speaker is of poorer audio quality. Our ears tend to favour the louder speaker.
The reason I'm saying this is because I know that the 901s are 2dB more sensitive than the 60s (or at least the Series IVs were). So the 901s may sound better to you because they will be louder.
You owe it to yourself to get an SPL meter if nothing else than to properly calibrate all your speakers. I suggest staying away from the Radio Shack meter because it's analog and you have to use multipliers. Get a digital one like Jakewash's or one like mine. By all means though, don't just trust the SPL meter. Use your ears for final tweaking.