How 'loud' should my Masters be, or how much compression should I apply before Mastering?

Delivering a ‘loud’ Master doesn’t necessarily mean you will get back an ‘even louder’ Mastered file or CD. The louder you compress your Masters, the harder it is for us to make any fine adjustments without producing unwanted side effects such as distortion or harshness and grittiness, or spurious level changes and rebalancing of the mix. We do have excellent Limiters and Compressors at our disposal and can apply this after any Mastering Eq adjustments to retain the highest accuracy and sonic performance. By all means send us a ‘guide level’ version of the track to indicate what sort of volume / compression / limiting you are expecting, as well as entering details in the Engineers notes section.
It is a bit like sculpture, in that it is much more difficult to make fine adjustments to something that is very hard, brittle and dense like granite or obsidian, whereas it is very much easier to produce fine detail with a softer medium like marble or wood.