It all depends on the sound you want and like. Best thing is to stick with your ears and your head for making things sound interesting than the specs of various diffrent boxes. Using your head, decent ability and knowing the strengths and weakness of your gear and exploiting them.

Stomp boxes are just as valid as a Effect processor and sometimes are a better tool for the job. I use a combination of vintage analog FX's like the tape echoplex, high end digital gear like the KSP8, and an assortment of stomp boxes ranging from the Electro harmonics to no name junkers. All have diffrent sound and work wonderfully together or individually. Best judge of the quality of an FX is if it sounds good to your ears.

Just try as many diffrent things as you can with your DX7 and buy the ones you think sound good to you and have fun, no matter what the spec, how old or new, cheap or expensive. If you like it and can afford it get it and have fun and always experiment to find new sounds with that gear. When you get something new try diffrent combinations of that with your older gear. There is no rule to any of this.

Try the TC boxes or the Boss VF1.

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Post indie electronic
Meriphew
www.meriphew.com

Once you hear a good reverb sound, you wont want to waste your money on noise boxes. The TC boxes are not quite as noisy as the others. I think it is mostly due to their higher threshold abilities. They do also however, raise the noise floor substancially. I personally find Lexicon to have the best for the buck, and Digitech gear to be the noisiest. Their rack gear is noisier than some floor pedals even. In fact I am forced to charge more for mastering when some Digitech gear is used because I have to go through two more stages of processing to lower the noise floor from the 20 dB that it raises it. Yup, 20dB rise in floor noise. Now does that suck or what!? Its not originated from the power supply either. Some people say, "I don't hear no noise"...what can I say?....buy it then. Hopefully I won't be summoned to process the work.
Don't shop for and trial equipment in a noisey guitar shop. Do it in a quiet area where you can hear all of the thing.
MORPH!

As far as I can see the best bang for the buck is accually the Kurzweil KSP8. It has the best small room I've heard. Also gives you tons of other FXs. You get a swiss army knife of bus options. You can do mono stereo or 5.1. The coverters are great and the thing is very quite. Distortions are certainly digital though. There are some things only it can do. Can be a mono synth if you like as well.
If you need a box to cover a lot of groud you will not find better on the market right now in my humble opinion. It is very very programable and also has a kurzweil learning curve though not as bad as the K series. I am very happy with mine.