I'm brand new to the RoundTable.

I have a Presonus BlueTube DP mic preamp that uses a 12AX7 in a dual channel configuration. There's nothing "wrong" with the unit. The existing tube doesn't have any visible markings, so I'm not sure about the OEM brand.

I wanted to try some different tubes to fine tune the sound and perhaps derive a little more gain. I'm looking for a smooth, not gritty/grainy, warm sound since I will be using this exclusively as a mic pre for a pair of AT 3035s. The unit has a 16V power supply and I assume that the tube is powered at that voltage (which would affect the choice of tube?). I'm looking for some suggestions, including using a slightly different tube - 12AU7, etc.

I have done business with BOI Audioworks before and have been very happy with the experience. I say this because I expect to buy from them and get matched triode tubes for consistency between the two channels.

Thanks to all for your help and suggestions.