I have a fully restored Hallicrafters SX-42 receiver, known in its day as a high fidelity unit. It uses a pair of 6V6's for push pull audio. I am using a matched pair of Groove Tubes. If I pick up some NOS, or old stock tubes, and "roll my own", will I notice subtle, or dramatic differences? Yes, it can tune FM, and yes, it does also have an external input.