What does 'tightly controlled' mean? Tighter bass? If so, you might want to think about the idea that while tight bass is often a lot of fun, it is a coloration not found in real life.
But to answer your question you can mix the taps without damage to the amp. This is because each of the biwire inputs of the loudspeaker have crossover elements for the drivers involved. So bass won't have a complete circuit on the 8 ohm tap while highs won't have a complete circuit on the 4 ohm tap- and your power tubes will thus see a linear load. Whether this sounds right ultimately is a different matter, but the amp should be OK with it.
But to answer your question you can mix the taps without damage to the amp. This is because each of the biwire inputs of the loudspeaker have crossover elements for the drivers involved. So bass won't have a complete circuit on the 8 ohm tap while highs won't have a complete circuit on the 4 ohm tap- and your power tubes will thus see a linear load. Whether this sounds right ultimately is a different matter, but the amp should be OK with it.