You didn't quite answered what I wrote...
I do have a 5.1 currently with an SVS SB1000.
Will using the AVR as pre amp to a power amp color the sound compared to using an independent stereo system (integrated or pre+ power)?
If I'll use the AVR as pre amp, will I get the benefit of a better DAC and amp on an external stereo amp, therefore achieving better stereo sound quality? Or do I need to completely bypass the AVR for such usage?
What do you mean exactly by bass management? Do I need a stereo amp that's 2.1 to use my powered sub?
What does pure mode matter to me? Is that the way to use the AVR as preamp and the external amp as DAC and amp? And if it is, then again - wouldn't it color the sound, which I want to avoid?
We know TLSGuy doesn't like AVRs at all so I think he is being very helpful in this case already.
I can offer a few comments:
1) Not AVR preamps/DACs are transparent enough to not color the sound but most are. To narrow down those that won't color the sound, if use well within their output limits, you can examine their audio specs, and the available bench test results.
You would want something has THD+N (total harmonic distortions plus noise) better than -80 dB, that is 0.01%, for the full bandwidth 20 to 20,000 Hz. Many may argue that even -60 dB (0.1% THD+N) would be below the threshold of audibility by most humans.
I am of the opinion that it is better to go with -80 dB of better because then we don't have to be too concerned about the harmonics contents. Also, if THD is as low as 0.01%, other distortions such as IMD (intermodulation distortions) would likely be low too.
2) External stereo amps don't have build in DACs, some mid range integrated amps do, though such build in DACs in such integrated amps are generally no better than those build in with good AVRs such as Yamaha, Denon, Marantz, Anthem's. Obviously the very high end integrated amps do have better DACs build in.
3) Bass management give you the functionality in setting crossover points such as 60, 70, 80 Hz etc., so that the low frequencies can be directed to your subwoofer and let the other speakers focus on the higher frequencies (from the crossover point and up).
For more information, try Wiki:
Bass management - Wikipedia
As others alluded to already, you most likely can just use a competent AVR without the help of an external amp.
To do some research on your own, a good place to start is to take a look of reviews that include detailed bench test measurements, such as those linked below:
Denon AVR-X3300W A/V Receiver Measurements and Analysis | Audioholics
Denon AVR-X3700H AVR Review | Audio Science Review (ASR) Forum
Onkyo TX-RZ50 Review (Home Theater AVR) | Audio Science Review (ASR) Forum
Yamaha RX-V6A 7.2 channel 4K / 8K Dolby AV Receiver Review | Audio Science Review (ASR) Forum
The AVR-X3700H measured much better than some integrated amps also tested/measured on the same bench at ASR.
On the external power amp question, you should use an online calculator to find out your actual power need first, then go from there. Again, I agreed with other, if I were to guess I would say you don't need one as long as you go with a mid range AVR that is rated for 100 W 8 ohms with both channel driven simultaneously.
Peak SPL Calculator (hometheaterengineering.com)