Can anyone honestly say they have heard a recording where their first reaction was it sounds bad due to using entirely the same preamp? You would have to go back to the days of the Beatles or Led Zep to hear such a thing and I don't have a problem with any of those recordings.
However, I have a heard a lot of amateur recordings where problems were one of the following:
- The user did not understand EQ balance among the instruments.
-The user had no clue how to use a compressor.
- The tracks had the wrong mic, wrong distance and wrong placement for tracking the source.
- Basic understanding of reverbs and delay were lacking.
- Proper matching of the mic and the preamp for a desirable sound was not done.
Sonically, having all of the same preamp would be the least of my worries if I hadn't covered the other basics.
On a pro level, most R&B, Pop, Country rhythm tracks are cut on a single console with the same preamps. The vocal and other key focus tracks are done on external preamps. The bass may also get its own treatment. This is generally, not always. In metal and rock it might take a different bent, API preamps on the drums. In rock however, there are mulitple layers of guitars so here, yeah, some variation on the preamps would be in order so things don't turn to mush.
If you’re on a NEVE, API or Trident console you don’t need to worry about the preamps, worry about the performance. If you’re on GC discounted gear where a single instance of a low cost preamp sounds bad to begin with, then yeah, the whole recording may suffer.