[CA] TM258 with Axiom?

[Beggers]

Hi,

I currently have the Axiom 9 and the RV 100 duper, but it struggles to find the bottom at times in 400-500ft using the chirp sonar.
Has been suggested to run a TM258 and a Y cable and keep the real / down vision and use this for conventional sonar.

Have looked and think i have the answers, but have been told you need the CP370 as the axiom won't run the TM258, although also been told it will (but only at 600w) it was recommended i try it and see before adding a CP370. This stuff is pretty expensive here in New Zealand so want to get it right.

Any answers would be helpful.

Thanks

Chris