Looking to upgrade

It really helps to know your price range, but I will assume a moderate budget. Regardless of what you have to spend, start with the source. BlueRay is stunning on the PS3 (when I was able to get it to work - I have since returned two of them in frustration) and if that is all you watch your in good shape. But if you watch mostly regular DVDs invest in a used Arcam DV 78 for around $500. You will be amazed at what this thing does for the price. It will be MUCH better for straight DVDs than the PS3.

After that, I would go for an Arcam AVR 350 receiver. Then I also would get a Furman line conditioner that is in your price range. You can find this on Audio Advisor if you don't have a local dealer.

After all this, if you still have money, look at upgrading your speakers.

Since you already are familiar with Marantz, you may want to consider one of their new A-V Receivers. They are affordable and are jam packed with features and power. I have the SR5600 which is about 1 year old now driving a 5.1 channel system with B&W loudspeakers (90 watts continuous x 7). I think this particular model is discontinued but they have newer ones to replace it but for the money, they are a great purchase. I love the B&W sound with their tapered tube loaded nautilus tweeter technology which absorbs soundwaves from the rear of the tweeter. This technology can be found in their 600 series and higher as well as in the smaller M-1 mini-theater monitors. Clean sounding is an understatement!! Go to www.bowers-wilkins.com. Just my opinion.

if you're comfortable with used and already thinking Lexicon DC - I'd endorse the DC-1 which I'm using. I also recommend B&K 5 channel amps. I had a AV5125 which sounded wonderful (have since gone to monoblocks - much more $). the DC-1 and B&K AV5125 can probably be found for $1100-1300. Make sure the DC-1 has version 3 or better SW, preferably version 4. Better still, the DC-2 is a little more $ and has more digital inputs. I don't think you can touch the sound quality of this combo with a receiver.

Nice system. However, DPL II and newer DAC's will make a nice difference. To improve upon sound, you will either need a pricey receiver or go to seperates. The other option is to get a nice DVD like the arcam suggested above and let it do all of the decoding for you.
Hard to suggest what without a budget. I like Anthem and Sunfire...definitely a step above marantz.
Do you have a line conditioner? Even an inexpensive one will make a difference.
Better cables? better video?