I was astounded when I started listening to the HD598s. They are so immersive, something that speakers really can't do unless you dedicate an entire room to it. That doesn't mean that I don't use my set of Cerwin-Vega floor speakers, however. Speakers are great for having people over but as soon as "quiet hours" come on the music goes off (from experience I have also learned that sub-woofers are pointless in college since the rattling walls cause your neighbors to hate you...). So for me, speakers are great for social occasions but these Sennheisers have made listening much different due to the kind of immersion you can obtain with headphones as well as the detail that is gained through headphones (noticing instruments and sounds I hadn't before). I wouldn't say that headphones have replaced my speakers but I do listen to headphones more often now than before.
Here is what I'd do. Buy the 598s and listen to them through your Denon receiver (that should make the 598s shine-I listen to mine through an old Pioneer receiver); before the phones are even burnt in you will likely know if they are worth having for home/private audio compared to speakers (honestly, when I am alone I prefer using headphones). If you don't like them, return them. Keep your Bose phones and use them in the library when you don't want to disturb others and need to attenuate noise (they definitely aren't crappy so there is no point in spending more money to buy crappy headphones).
I am getting the vibe you just want to try something new which I fully support; I was doing the same thing about 4 months ago when my Skull Candies broke and I bought the 598s used from a friend as "replacement" headphones. I flatlined listening to the 598s for the first time (maybe an exaggeration, but I'd never experienced music like that before). If you are like me I imagine you will be pleasantly surprised upon listening be it the 598s or whatever else you choose. Check with other Head-Fi members since, obviously, I am pretty new at this.