A down sleeping bag may cost well north of $300, but this much can be said about it: With reasonable care it could well last the user聮s lifetime. I have a Feathered Friends Gore-Tex Swallow (www.featheredfriends.com) that聮s some 20 years old and is still in great shape.
Feathered Friends Swallow sleeping bag

So yours may not be fading yet. You say you keep it clean. But have you had it laundered? One of the things that can cause the down to clump and lose loft is body oil that gets through the bag liner and into the down fill. Soil and crud also can come in through the shell and stitching. You can ship the bag to Feathered Friends in Seattle and they聮ll wash it for $35.
Or, buy some Nikwax Down Wash ($9; www.nikwax.com) and take the bag to a Laundromat or anywhere with a front-loading washer (the agitator in top-loading washers will tear the baffles). Wash according to directions on the Nikwax bottle, then dry the bag on medium heat. Some people advocate tossing in a clean tennis ball or two to break up the down clumps. But the tumbling action does a good job of that, I think. My belief is that you聮ll see the bag聮s loft restored to at least 90 percent of what it had originally.
If not, well, maybe the down clusters have gotten crushed a little and lost some of their resilience. That聮s not fixable. So what to do? If you聮ve liked the Swallow聴a fabulous bag, in my view聴then maybe get another one. Rated to 20 degrees, the Swallow is cut fairly comfortably yet weighs only two pounds. A new one with nylon taffeta shell goes for $349; upgrade to waterproof/breathable eVent for $404.
If you聮ve found you really haven聮t needed the warmth the Swallow provides, you might consider a ($249; www.marmot.com), a lightweight down bag that weighs one pound, eleven ounces and is rated to 30 degrees. I聮ve had an Arroyo for several years and have found it great to the freezing mark or a bit lower. And I love its light weight and compressibility.
If you聮d like a bit more warmth, then Western Mountaineering聮s Apache Super MF ($375; www.westernmountaineering.com) is the ticket. It聮s rated to 15 degrees, and I love its excellent fit and polyester shell, which is breathable and naturally water repellent. Weight is outstanding at two pounds, one ounce in the regular length.
You聮ve got your winter gear, now get outside and use it. makes it easy to find nearby slopes just begging for fresh tracks.