2 channel setup - Should I add a center channel or $1500 subwoofer?

Really depends on your priorities. Is this a home theater under construction or a audio system that you want to enhance over time to also be a home theater. If audio is your priority... a sub. If HT then center channel.

Something like 85% of HT sound comes out of the center channel. So from a HT perspective really important.

My opinion is the center channel.  Even though movies will work just fine with only left/right, there is an increase in clarity when you have a dedicated center channel. 

It is already apparent that there are going to be a difference of opinions here.  A subwoofer will make the effects/explosions much more visceral.  I suppose it depends on which element is more important to you.

I say buy 500.00 worth of room treatment, and TWO 500.00 subs and get 3 X the return.. The center channel.. Looks good, I guess. But any OK pair of speakers will image a center channel BETTER with regards to music.

That center channel is for voices really and actually the most important speaker in a HT set up.. You could actually do away with stereo speakers and add subs, ONLY.. Some time simple is really really good..

HT and music can work together, the best IMO is stereo with at least TWO subs and some room treatment.. And not the other way around..
Leave the center channel OUT, you already have a great phantom center..

Room treatment FIRST.. 500.00 You’ll be amazed with just that, then those 500.00 subs will sound like the single 1500.00 you planned on but X 2 at 1/3 the price EACH.. You can’t gear your way out of 50% of the sound, THE ROOM...

Master M will chime in.. He'll certainly tell ay.. :-)

Regards

I'd recommend a center channel over the subwoofer if you're going to have more than one person watching movies/TV and if your main left and right speakers are far enough apart (i.e. your display is large, or you are sitting close to it) that the people watching will hear things coming from the left or right instead of center of the screen. This is usually more distracting than a lack of bass output.

A center channel won't necessarily improve the clarity of dialogue or other sounds, especially if the room is very echoey or if the center channel ends up needing to be placed inside of a cabinet or in some other way that creates additional reflections or increases distortion. Room treatments, or at least filling up the room with stuff, is the best way to improve clarity.

Better off with four $350 subs. IF you have the space. Unless you prefer lower Hz in one listening spot.