Unsound, almost any tube preamp manufacturer has to contend with transistor power amps that have a lower input impedance. Even the old Dyan Pas 3X was an example of that- the X designated that it was set up to work with amps with an input impedance of 10K ohms.
And of course these days there are tube preamps with balanced outputs too. The one we make has been in production longer than any other - and it has direct-coupled outputs so the 10K input impedance of transistor preamp is no worries at all.
And of course these days there are tube preamps with balanced outputs too. The one we make has been in production longer than any other - and it has direct-coupled outputs so the 10K input impedance of transistor preamp is no worries at all.