How to bring headphone amp into a stereo system ?

Hello, I am considering getting some entry level Hifi headphones (possibly Sennheiser HD660s). Though this might not come right away, I am willing to complement it with headphone amp as well. I already own an entry level 2ch system with an integrated amp (Sony STRDH190) and two sources : network streamer and turntable. My question is this : what is the best way to use both sources and let me easily switch between the speakers and the headphones? Using the front microphone output of the poweramp would work, but I haven't find any technical specs as to wether it would drive enough power for the headphones I'm looking into. I understand it might also be a bit stupid to buy such expensive headphones to listen through a possibly cheap component of the integrated amp. Hence I am considering headphone amps as well. But from what I can tell, I would either need hook in between one of the sources and the integrated amp (and give up on listening to the other source through the headphone amp) OR get a pre-amp which will play the role of input selection (and possibly dish the integrated for a dedicated power amp). Looking for some examples of how this situation is usually solved. Thanks!

That receiver won't allow for easy integration as it has no variable or fixed level RCA output. You could get a set of splitter cables for your sources but that would require a headphone amp with two inputs and you would see a very slight loss of signal voltage (probably a non-factor). My setup is as follows: Phono SE output to receiver/processor, XLR output to Cayin iHA-6 DAC SE output to receiver/processor, XLR output to THX AAA 789 *Receiver Zone 2 RCA output to THX AAA 789 SE input, SE Passthrough to Cayin iHA-6. Used for other sources hooked to receiver/processor that do not have a direct HP amp connection. When listening to CD, SACD, BR-Audio, etc... I have an extra set of XLR cables ran to the HP amp area and hook-up as needed. Same as the others, SE output is ran to the receiver/processor. *My marantz can downmix surround HDMI sources and output them in 2-channel via the RCA analog outputs, not every receiver can do this for all input types.