I have had the Mullard CV4003 paired with my TS 5998 for the last week and I must say that my search for a 12AU7 is over. Its combination of smoothness and transparency can't be beat, IMO. It doesn't take away from all the things the 5998 brings to the table, but rather enhances them. It rounds the edges off the 5998, which sometimes could be bright paired with other tubes. It is a surprisingly quiet tube as well, and allows you to hear through to the recessed layers of tracks. Lovely.