Multi-band audio delay filter? I'll be processing stereo music signal. What I need is to first split the frequency range into 2 bands at 50-100 Hz. In the higher band, I need to delay one of the channels by a certain number of milliseconds. The lower band has to be delayed as a whole (left+right channel) by a different value. Is this doable? Is this doable in real-time (think VST plugin for an audio player)? If it's possible, can you please give me pointers (algorithm-wise) on how to implement it?

P. S. I need to preserve the sound quality. If such a filter would ruin it, then my idea goes to the trash can.
Not sure about audio plugins but what you want to do is certaininly doable by using filters (delay line in your case in the subbands. Split the band using using a Perfect Reconstruction Analysis filter bank. Now plugin the delay line and then combine two subbands using synthesis filter bank. Note that you need to be extra careful while plugging in the filter between analysis and synthesis filter bank. You can check some of these papers for ideas on how to do subband filtering.

https://dl.acm.org/citation.cfm?id=2817121

http://ieeexplore.ieee.org/abstract/document/1661283/