Volume control knob position does not tell much, other than in general you would try to avoid turning it higher than 12 O'clock, but it really depends on other factors such as the input signal voltage that in turn depends on the kind of device upstream and the source media contents obviously. It also depends on whether the volume control follows a linear scale or log scale? I think most vintage integrated amps would have the linear one. Your A9010 isn't that old and it use an IC to do the job, so I think most likely it has a log scale characteristic.
If it is log scale, you will notice that the level would increase more gently as you turn it up but will increase more rapidly (more sensitive) as you keep cranking it up.
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That preamp, to me, is a low cost nice toy, but if your goal is transparency, neutral kind of "sound signature", then yes you are better off with an integrated amp, AVR, or a stereo receiver.
Much more expensive integrated amps such as Yamaha's (anything
above the A-S801), likely offer cleaner preamp output signal and better audio specs from input to output, but can you tell the difference between 0.01% (that's -80 dB) and 0.001% THD+N, I doubt if even 1% of home users can, unless they live in an anechoic chamber with every device in the signal chain perform at the same level, and the best speakers are used.
More often than not, the likes of the AVR-X3600H, SR6014, RX-A1080 and higher models actually have the critical part such as the preamp volume control, DAC and OP amp ICs that are superior in audio specs, relative to the likes of the entry level integrated amps such as the Onkyo A9010's. You need to go up to at least something like Yamaha's A-S701 and higher models, to match such AVRs in audio specs.
Advantages of the AVRs mentioned are 1) better price to performance ratio, 2) flexibility, example: more inputs, you typical get a few analog, at least 1 optical, coax, USB, and of course HDMI and video capability so you can see what's going on (such as volume/level), 3) Automatic REQ feature so you don't need to bother with those minidsp that costs money, add wire clutter, and don't always do any better. For the same money, the last year model AVRs typically offers such good discount that I dare say 100% of the same you end up with a more powerful unit than a two channel integrated amp. With C-19 going on, there appears to be a shortage of AVRs so I assume it is not as easily to find such discounts. But if you can get something like the AVR-X3600H for under $1,000, it would be a better buy than an integrated amp such as the A-S801.
The R-N803 is a nice unit that I would not hesitate to recommend, but it does not have higher output, the difference is in the multiple decimal points of 1 dB so practically though, so they practically offer the same power output into a 4 ohm resistor load, and that's based on bench test measurements of the X3500H that has the same power amp and power supply as the X3400H. The X3600H beats the R-N803 in dynamic output but again in practice they are equal.
One more important advantage of the AVRs mentioned is that it has multi-channel preamp outputs so if and when you need more power you can connect the pre outs to external power amplifiers. The R-N803 does not have preamp outputs, only line outputs. That, to me is a show stopper but if you are sure you won't ever need more power then obviously it is not an issue.
1) No, you need subwoofer outputs (, not line outputs unless you are prepare to add an external volume control for the subwoofer.
2) Most AVRs do have bass management features and Room correction, most integrated amps don't, but a few do. Most stereo receivers don't have REQ, but the R-N803 does.
3) If you are referring to HDR10+, I believe you should be safe with the 2020 models, you should check the individual manufacturer websites just to be sure.
