The Yamaha FM synth are quite strange machines.
People judge them harshly today because probably ignore that, when they come out, PCM based synths had yet to come and the majority of the analogs sounded huge, but more or less tended to produce the same array of sounds; I think people were astonished by the metallic punchy sounds of the DX series, by the Electric pianos or by the infamous Synth Bass sound that was a trademark, together with the LinnDrum, of the mid 80s pop/disco productions. And probably people found the DX to be fresh air in a synth market dominated by really expensive and heavy analog beasts.
Another winning factor back than, was the velocity keyboard action, which produced great effects because of the FM synthesis ability to really change the harmonic content of a sound as you play the keyboard harder. Analogs couldn't produce the same results.
Of course they had no onboard FX and were "just" synthesizers, so they were almost killed in the D-50/M-1 era, dawn of the PCM based instruments.
The early models had worse D/A convertors, so they have a grittier sound and are more noisy.
If you look for a cleaner DX sound you need to go on a DX7 mk II or a TX 802 (multitimbral!)
If you look for THAT 1984 sound, you need to go on the first DX7 or TX7 or TX 816 (a studio classic)
I got a TX7 for 60 euros, un-programmable from the front panel, but programmable with a SysEx computer editor. If you add some classy effects (like a very good reverb) it can be really surprising.
As to the RV4 bass sound, I remember that on the old forum (Jarrenet) the initial idea of the DX100 being used with the "Lately Bass" sound, was shown to be wrong.
I read in some forum that the RV4 bass sound should come from the Casio CZ5000, although I think it could be also a Synthex sound.
The DX100 IMHO was used on the choruses for the metallic effects that double the main synth line.
Good luck in your choice and consider that, in any case, a DX7 mk I or II has still a wonderful keyboard and you can just use it as a MIDI master keyboard (better in its feeling that any 5 octave modern USB keyboard controller - weight explains also why...)
) tape only once the manufacturer has allowed the band to show their trademark. The classic though is Ronald Mael of Sparkes labelling his Roland keyboards (actually, I think I may have seen him with a few other makes (Yamaha perhaps?) labelled this) over with Ronald. 
