Pro Tools LE + Bose Speakers > high pitched noise Problem

[sandmansndr]

Hello I was wondering if someone could help me out:

Like the title suggests, my set-up is Pro Tools LE with an Mbox MINI 003 which I've connected to my Bose Companion 5 speakers thus:

MBOX MINI 3 >
2x male Jack to male Phono cables >
2x Phono female to female converters >
1x 2 male Phono to 1 stereo male mini-jack cable >
I've connected this last cable into the external input of the Bose volume control.

This seems to be the only way how I can hear Pro Tools through the Bose speakers because the program doesn't recognise the Bose speakers as a valid hardware to run the audio through. This way the Bose speakers are used the same way as any studio monitors would be used.

My problem is though, that through this set up I hear a high pitched buzzing noise (B6 on the piano) once the volume goes past around 40% on the Bose speakers and I have Pro Tools open. once I disconnect the cables between the Mbox and the Bose speakers the sound stops. The sound also stops once I close out of Pro Tools.
It doesn't interfere with my work much- but it's definitely a nuisance.
Would anyone know what this could be and how I can fix this?

Thanks very much,

Sander