Go with the claret hose, they will not overpower the manx tartan. If you are up for another jacket/waiscoat combo, may I suggest poacher tweed, green with a subtle windowpane check in claret/oatmeal. I have one which I wear with MacLaren and Paterson tartans (both predominatly blue/green tartans)

Link to poacher tweed material.
http://www.scotweb.co.uk/tartan/Reiv...ingbone/100128