I have a Sony receiver STR-DG1000 and the Sony amplifier TA-N9000ES. I read here:

that the amplifier will provide great sound either at 5-channels or 2 BTL.
After I made the connections following the manual instructions the sound level and sound quality are exactly the same as before without the TA-N9000ES. I switched to BTL mode and I've got the same quality. I am using B&W Series 600 speakers.
Am I doing something wrong? Is the amplifier made like that? My receiver doesn't need that amplifier? Is the amplifier broken even when I get sound from the 5 channels? Any advise, opinion will be appreciated.
Thank you.
What differences were you expecting? I doubt there would be any major ones as long as the amp is operating properly.

I'm with Kal - why big differences? The extra power you get in BTL mode may be overkill for your small monitors - you may be already pushing the limits of what you can eek out from them. Speakers compress when driven really hard and will start to sound dull - up to a point adding power is helpful especially in the bass response (but if you use a powered sub then you may not notice this) and then it quickly becomes diminishing returns on small monitors.
Can I bi-amp the 5 B&W speakers using the N9000ES & the DG1000?
Thank You!
Possibly but (1) you will have to figure out some way to balance the two amps and (2) there's little gain (!) in doing so.

I would expect the sound quality to be similar as they are both Sony pieces. Utilizing an outboard amplifier with a receiver essentially turns the receiver into a processor. Unless there is a massive amount of power difference between the 2 units there will be little difference in volume if I am understanding you correctly. Be cautious when looking to bi-amp any of the speakers. Check the power rating on the B&W's.
I own the TA-E9000ES and TA-N9000ES. I don't think you need to run biamp configuration as your TA-N9000ES would provide sufficient power to 5 channels in your 600 series B&W speakers. I believe your Sony receiver has outputs to hook up your TA-N9000ES.
I've been listening carefully on the last days same recordings on different formats Lp, CD, SACD, cassette, MD. The Nutcracker, Pink Floyd, Miles Davis.
Yes! there is a difference after the N9000ES was connected to the DG1000. While using the Automatic Format Control the sound was the same (weak). But when I switched to Analog Direct I noticed more volume, depth and great separation n any angle. The SACD played at 5.1 improved but definetly the 2-channel analog direct is better now. The standard volume level for my apartment was -15bd but now I have to keep it under -25db and I can still hear some bass.
I also noticed before that playing films from Columbia Pictures or Sony Pictures using the Cinema Studio mode the sensation is like if I have several speakers all around than when I use Dolby Digital alone.
Thank you all for your opinions.