RE: nikko seawolf
All of the ones I am aware of are on 27 MHz, with several different bands available (I think 6). I also think that some later models had interchangeable crystals. These subs have a proprietary "all-in-one" electronics boards so it would not be easy to change out the RX.
It is a fun RTR sub. But keep in mind that there are no more spare parts available, at least from Nikko.