Creating sounds from scratch with a DX 7 is much harder than starting with a preset that is close and only needs to be edited. It is a good way to start learning the process of programming DX sounds. You start to figure out what the carriers and modulators are all about and how they effect the sound. There are no filters in a DX so the sound of a filter is being created by the the fact that harmonics are being altered in a way that a filter does. 

What is cool about something like FM7 or FM8 is that there is an easy edit mode which is quite incredible actually. You can take a sound for example and do an easy edit on an imaginary filter and as you move the slider you hear the sound change like its going through a LPF. What is really happening is the modulator levels are all being tweaked at once to create the same effect. Also easy edit mode allows you to change the ADSR envelopes fast and all at once too which is also rather cool.

Yes you can use two DX sounds which will fatten up a sound big time. So a DX/TX combination works well here. You can also set the TX so it just mirrors the DX sound you call up but you set the tuning of one say at -6 or -8 cents and the other at +6 or +8 cents. This will create a fat detuned sound. And if you are in stereo you can hard pan those two sounds for a very wide final sound.

 The other day I walked past a charity shop and there in the window was a Kawai K5 rack synth for 50 bucks. After some contact cleaner on the switches, a new backlight and invertor for the display (about 10 bucks), it's good as new. Can be picked up for peanuts. A 64 oscillator additive (not FM) synth but produces a marvellous range of bright, digital sounds that sound very very 80s. Great fun. Of course, my favourite synth is my much treasured Yamaha FS1R. I bought it many years ago from a guy who decided he wanted an Access Virus instead. It's the one thing I could never bear to sell. Truly a unique device. You need the (free) editor to manage it though, programming from the front panel is a nightmare. But with the equivalent of four full 8 operator FM synths plus noise generators plus formant synthesis, it can make the most amazing sounds. Never been recreated as a plugin.