No sub installed. But as Skyblazer noted, there's definitely increased bass output in the STEREO & UPSAMPLE modes compared to BYPASS - even for just the front 2 speakers. Since I don't have a sub installed, and the sub option is set for NONE, there should be no bass management processing when only listening to the front 2 speakers. I understand that in surround modes, the bass from any surround channel including center is redirected to the fronts based on the x-over settings for the surround channels. However, for just front 2 speaker stereo operation, the frequency response should prettly much stay the same whether in STEREO, BYPASS, or UPSAMPLE modes. I'm in my 3rd week with the 990; I'm really impressed with the sound quality, but there are obvious usability and processing execution issues. I've got another week to decide whether to keep it, and I'm quite torn about it.