Welcome to the forum!
I have owned a Yamaha PSR-E433, which is a predecessor to the E473, for about 9 years now, and I have been very happy with it. While I don't yet play professional gigs, I have been playing keyboard for over 44 years, so I am no novice.
I always tell people that, unless money is a concern, to get the E473 over the E373, because you get so much more, including the live control knobs, more effects options, more registrations and better access to them while playing, and a USB-to-device port.
So, to answer your questions the best I can...
Speakers -- I see no reason why you would not be able to turn down the volume as low as you want on either keyboard.
Keypad -- I believe the key feel of both of these keyboards is similar and of decent quality, though it has been a while since I actually played them. This is one area where there is a kind of consensus that Yamaha put a superior keyboard on the E433, but then put slightly "degraded" keyboards on their later models, and that is one of the reasons that I've kept the E433 so long. But the newer keyboards still feel fine and, in my opinion, feel better than any of the recent Casios that I've tested. However, I have yet to play the newer Casio CTS-500 and CTS-1000V models, so I cannot comment on those.
Live control knobs -- You do not have to worry about those. They are read in a digital fashion and will not affect the sound unless moved/changed. So, for example, you can select a sound, set the knobs to control the filter, and then set some extreme cutoff and resonance points with the knobs to get some crazy synth sound. But then if you select another sound on the keyboard, that sound will load up with its default, factory settings and the keyboard will NOT apply the knob settings to that sound unless and until you actually adjust a particular knob. Similarly, you can save a combination of sounds (main voice, dual voice, split voice) along with various knob settings and DSP settings to a registration. When you then call up that registration, it will play exactly as you saved it, regardless of what the current settings of the knobs are, and the knobs will not affect that sound, again, until you physically change the position/setting of one of the knobs.
I will say that those knobs are worth their weight in gold, as they really help you easily customize a sound and bring synthesizer-like features, such as a filter, to such a relatively low-priced keyboard.
Feel free to ask any keyboard related questions here!