I have 3 “power amps” that have input switching and volume control. The volume is an attenuator, not an active preamp.
Nope -- a level control (or two) has been quite common in power amps for at least 60 years. Some -- like my old Kenwood KM-8002 from back in the day -- also had switchable line level RCA inputs and speaker selector switches!
All modern amplifiers worth any look at have speaker protection relays for power on / off thumps and DC detection. I have never heard anything of the sort from my LA90 and managed to even trigger the protection a couple times with overloads. Worked great.
Correct, most power amps dating back to the 70s have an attenuator on the left and right channels to limit max gain or balance the channels, speaker selectors etc. They can be hidden on the back sometimes also.
There is no input to directly access the power amp section, so it is not a "power amp," the end. OK yeah you could plug a preamp into an input and set the volume to some set level and use it like that, I do that with an Aiyima fed by a Bluetooth adaptor in one little setup. But that's not a "power amp" in the traditional sense. Does this thing have a remote? It doesn't seem to, which makes it useless in my book.
The separate preamp is for dual bridge amp and remote control... Not because the la90 is not an integrated.
The preamp in the LA90 (D) is rather formal. If you need a high-quality relay volume control, then they suggest using an external preamp. Even in tests, Amir turned off the LA90 (D) preamp and performed measurements. Most likely, this was agreed with the manufacturer. If you conduct a survey here, then most of the owners of the LA90 (D) most likely do not use a preamp in it.
The preamp on the LA90 looks logical, as its presence increases market coverage. On the B200, they wanted to keep the input switch for the same reason. Although most of the LA 90(D) and B200 users use only one input.
Topping's approach with these two amplifiers is actually very far-sighted and clever, and other manufacturers could learn a thing or two from it.
Nothing basically. The device is totally transparent, so it will not do anything to the sound.
It's all about you loud the signals come in, and how loud you want it to be at max volume. If it goes loud enough with the low setting, you should be good. Otherwise, choose high.
Is the single amp getting loud enough? If so, you don't need a second one. If not, the next question is if you can drive the amp to maximum power with the maximum voltage of your DAC? So let's see: DAC output is ~4.1 V, and the gain is 10 or 20 dB, which makes 13V at 10 dB, and 41V at 20 dB. The power supply seems to be 64V, so that may appear to be much more. But looking at the maximum power of just 56W and 36W (into 4R and 8R), it seems more like the limit is here the 4A power supply. So you should be fine there.
Beyond the answer from @voodooless, the other thing some preamps will give you is EQ. This could be basic tone controls, 'loudness', Quad's slope control, all the way up to full room and speaker correction and subwoofer crossover. The full correction in particular can make a significant difference to sound quality.
Then just get an AVR and be done with all the audiophile nonsense
Then don't buy broken stuff in the first place. A DAC should not have a high output impedance.
Still no difference even with a (rare) high dac output impedance - as long as the reactive impedance of the load (including interconnect) is sufficiently high not to cause frequency response variations. It would have to be a very high output impedance, and a very capacitive interconnect for that to result in audible FR changes. Even then - if you use a preamp with low output impedance, it only helps if the reactive impedance into the Preamp is higher than the amp. And if you are using the same interconnects out of the DAC, then it doesn’t help.
