I would try Amperec tubes, but I'm not sure how much of an improvement in this area you will gain with a different set of tubes.
But I think you will get what you want with a different preamp. Swapping tubes may be cool, but I kind of doubt it will be a long term solution.
But I think you will get what you want with a different preamp. Swapping tubes may be cool, but I kind of doubt it will be a long term solution.