Receiver Too Weak?

Your receiver is low end and does not come with pre-outs for the fronts/center so you can run them to a powerful external amplifier.

What is your budget?

Get a processor with pre-outs (price can go up quickly).

Here is some affordable stuff

https://emotiva.com/products/basx-mr1l-9-2-channel-dolby-atmos%C2%AE-dts-x%E2%84%A2-cinema-receiver

Or

https://summithifiusa.com/collections/tonewinner/products/at-300-16-channel-pre-pro-releasing-soon

Pair such a processor with one of these amps.

https://emotiva.com/products/xpa-5-gen3

https://www.monoprice.com/product?p_id=15593

https://summithifiusa.com/collections/tonewinner/products/ad-7300pa-amplifier

If you have a slightly bigger budget...

https://www.crutchfield.com/p_642AV10/Marantz-AV-10.html

https://www.crutchfield.com/p_440B15852B/Rotel-RMB-1585-MKII-Black.html?tp=180

etc

If you have an unlimited budget...look at Trinnov, Storm Audio, etc.

On second thought, yamaha receivers do offer some features like dialog level, dialog lift (if your center channel has suboptimal placement), etc after you get into their Aventage line. Hence, the minimum you should get is the Yamaha RX-A4A Aventage model. Such receivers have a bigger power supply and current to spare for your B&Ws. If it was still inadequate, it will have preouts for external power amps depending on the number of channels you’re running...it may even turn into ’hifi’ hometheater depending on the amp and power conditioning you chose.

Also, bump your center channel level a few dB up, as someone else suggested (and as needed). Set the cal mic right at ear height on a tripod. 

On a general note, if you have your center channel laying on the floor or stuffed in a cabinet, etc, it is not a good idea. It is also a good idea to have 3-way concentric driver designed speakers with a lower woofer/midrange crossover for center channel applications.

The floor speakers are bi-amped and the setup for it in the receiver settings is done. Should I undo that? If I understand correctly that would reduce the power which will make them perform worse, but maybe the balance will be better until I can improve my equipment? Or does bi-amping not really work as well as it sounds like it should?

- Remove the "biamp" ....not the right thing to do with a low/bottom budget receiver.

- Read the receiver manual front to back. Learn how to setup and run a calibration correctly...and understand all features.

- Read some of the comments above again if music is a priority. For just TV and games, don’t worry yourself too much..do whatever...