Tube recommendations

@chakster  - thanks for the info. So could I replace my 3 tubes with Telefunken ECC801s or would I need to replace exact type for type?

You can replace all ECC81 tubes with ECC801s or 12AT7 or 12AT7WA (they are all one type)
But all 3 must be one type like at matched trio 

Oh right. So what's the significance of the numbers 81, 83 & 88?

Never used ECC83 or ECC88, these are different type of tubes than ECC81, they are NOT interchargable and this is all you need to know. 

Dear @infection : """  As I'm new to tubes which tubes would you recommend to be a significant upgrade? """

Why so fast did you decided to change the original tubes?, this is as when you bougth a cartridge and one month latter you want to up-grade it or change it. Something must be wrong down there phono stage.

R.