The tape-out, tape-in, and use of tape monitor is required for AVRs which do not have pre-out and pre-in in order to use the EQ to get proper sound.
If you skip that step: It should not make the speakers quiet; it should just make the frequency response off.
Do you have the 901 equalizer hooked up to the tape out and tape in jacks? It sounds like you may not be configured correctly there and are hearing only crosstalk.
Try not using the tape monitor function. Just run them as normal speakers. You should hear at normal volumes? Do you? If so, we know where we need to be looking (at how you have the EQ attached to the tape connectors, and whether you have it powered on or not)
This is all much simpler if you use an external amp or (like my Poioneer 9700) have actual pre-in plugs on your receiver.
