How to connect multiple sets of speakers?

Since your AVR has 7 channels of amplification, connect one pair of speakers to 'speaker B', one pair to surround L and R, one pair to surround back L and R.
Using '7.1 channel stereo' mode to activate all speakers.

@ksmcalley - Do the speakers on the "A" channel sound OK? Do the speakers on the "B" channel sound OK if you are only playing one pair of those speakers at a time by un-selecting the other pairs of speakers using your speaker selector? What if you play 2 pair of speakers on the "B" channel? Is it only when you play all 3 pair of speakers on the "B" Channel that you have the issue?

Is this a "whole house" music setup where you have the 4 sets of speakers in 4 different areas of your home? Please explain where the speakers are located, and how you use them. It may help with recommendations. Also, your Denon AVR-1905 is a multi-channel home theater receiver. Are you using it for multi-channel HT in the main room? In other words, do you have a 5.1 channel HT setup (5 speakers plus a subwoofer - L&R front, L&R rear, center channel and a subwoofer) in the main room and use the speakers on "A" channel as the front L&R speakers? Or are you only using a single pair of speakers in the main room?

Here ya go!!
https://geoffthegreygeek.com/ufaqs/whats-difference-speakers-series-parallel/

Search series and parallel resistance. If you know the ohms of each speaker then you'll be able to use the formula to figure out what combination will get you the total impedance you want, up around 8 to 16 ohms. Or at least up to 4. You may have to go series on 2, parallel on one, something like that. You don't need a separate wire to each speaker. Its okay to run one wire speaker to speaker. My subs are like that. Its the way they're wired- in series or parallel- that determines the total impedance.