Help reinvigorating large dracena marginata?

Greetings all!
I have a large pot (huge actually...over 2 ft. wide and over 2 ft. tall) with a bunch of drancena marginata, some of which are over 8 ft. tall. This pot is a huge focal point in my house, so I'd really like to keep it going, but there has been a general decline in the apparent health of these plants for a while now.
Basically, the old fronds hang around for a long time before they turn a bit yellow and drop off, but the new growth doesn't nearly keep pace. The new fronds seem noticeably smaller than the older fronds. It is now getting to look like a bunch of long trunks with feeble clusters of short fronds at the ends.
If I had to guess, I might say the problem is that the amount of light is just a tad too low, but my wife mocks me when I make this suggestion, pointing out that our house is notoriously bright, with spectacular amounts of ambient light. I must agree with her on this point -- while the dracenae in question get little direct light, they do get what seems to me to be a pretty good amount of general ambient light.
Other particulars: The plant is definitely not root bound. The soil is one of those pre-mixed blends for indoor plants that you can buy at nurseries hardware stores, and the soil has been amended with a new bunch of this soil mix recently. I don't think I overwater the plants because I don't see too many suspiciously yellow tips. I used to ferilize the pot with a very light dose of liquid cacti fertilizer (because I do have tons of cacti), and this fertilizer was so weak that I could use it every time. Over the last year, I have changed my fertilizing system a bit -- I use Miracle-Grow maybe 25% of the time, and the other 75% of the time, I don't use any fertilizer at all.
What is an appropriate fertilizing policy for dracenae marginata? Is there any way I can force branching from the long trunks I now have? Should I cut off a trunk, and try to re-root it in hopes that new branches will emerge from the cut? Does this "diminishing lenghth and robustness of new fronds" pattern suggest any particular class of problem?
Thanks in advance for any suggestions!
Chuck
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snipped-for-privacy@yahoo.com (Dolchas) wrote in message

I put both my marginatas outside for the summer, and give them one inch of manure and a few earthworms to mix it in. My other 28 plants get the same treatment. They take direct sunlight surprisingly well, and that is how they make enough food to get through the winter months.
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