First I would forget the two unit limit and get a separate keyboard. That will really open things up to what you can get and what you can do. Then you have to decide on what it is that you need. The A4 gives you 4 monophonic parts or a polyphonic channel with up to 4 notes. Is that really enough? If not consider the Digitone. Another thing to consider is do you want to use the new unit to make loops to use on the OT? If so, consider the AR. They seem to be made for each other. The AR can make your percussion loops, then convert to synth loops for bass lines and arps.
Yeah I guess I may without the built in keyboard.
But the AR is of no use to me…all drums are done by the OT…and if I get the A4 it can deliver analog drums.
@AdamJay while I think the Hydrasynth Explorer is a great device, sadly it is monotimbral, right?
Yes it is mono timbral. Only the forthcoming Deluxe is bi-timbral.
I typically sequence and sample my bass patches into the OT, then sequence the lead and tweak it live to overcome this limitation on my HS KB.