The negative numbers in volume control express signal attenuation. In other words, the volume control cannot make things louder, only quieter. So, the volume is expressed in decibels -20 meaning that the signal or volume has been lowered by 20 decibels. -15 means that it was lowered only by 15 decibels. Therefore, -15 setting is louder than -20.

I hoope that made sense.

It has limited value to you. If you try different amplifiers with your system, you'll see how the volume setting will be different to get the same loudness out of your speakers. It is also helpful if you're trying out different components in your system and you want to volume match your listening sessions to make sure you're comparing the components at the same volume.

Measuring in "dB" means nothing. Those "dB's" are relative to the input level of the source and that level varies from source player to source player. If you changed any component/amplifier/speakers within your system other than the pre amp, the numbers would very likely change for the same loudness level within the room. Then you'd have to figure out what number related to what component, and so on and so on and so on ...

Your CD player has a different output level than your turntable which has a different output voltage than your tuner so the pre amp always sees a different input level as you switch between sources. If there is more voltage into the pre amp on one source, the "dB's" will be lower because of the higher source output voltage. If the source has a lower output voltage, the "dB's" will need to be increased to achieve the same relative loudness in the room that you had when listening to the higher output voltage source. Make sense?

Use the numbers only as a reference for how you like to listen just as if the numbers where from 0-10, A-Z or the names of your pets listed by age. If you listen to "Flipper" for CD's then you might listen to "Lilbit" for your turntable. You get to make up the reference designation but it would only be for your benefit. You still turn the volume control up high enough to reach a satisfactory loudness level no matter what the readout states.

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It is also helpful if you're trying out different components in your system and you want to volume match your listening sessions to make sure you're comparing the components at the same volume.

Making accurate level matches while switching between components really should be done with something more accurate than an ever changing, never correct volume control readout.

Since most vc's attached to a digital readout are the sort where you can rotate the knob several times and still get increasing numbers on the readout vs. the old fashion potentiometer that runs from 7 O'Clock to 5 O'Clock about the only useful feature of the digital readout is telling you when you first fire up the system that +10dB is probably too high a setting if you're going to hit "play" on your CD source. The readout is a signpost and not a map.

I'm not familiar with the sound of either amplifier so I cannot comment on what you might expect with this combination connected as a bi-amp with your speakers. If the speakers accept bi-amped inputs, the way you have the connections made seems reasonable.

The preferrable set up for bi-amping is to use an outbaorad crosover, usually an active device, to split the frequencies in front of each amplifier. This arrangement lowers the demands on the amps and exspecially so on the amp driving the high frequencies. That amp no longer has to deal with the demands of reproducing low frequency information. In many ways this is one of the true benefits of a bi-amped system.

The use of passive internal crossovers is more suspect in its advantages and not all speakers marketed as bi-amp capable actually deliver the goods with this sort of set up. You'll have to decide how much benefit you hear from your current set up. It's very likely you would hear the same advantages with a simpler biwire connection with one very good amp driving the full range of frequencies to the speaker.

Your wiring set up seems fine to me. I don't have anything to add to that.