This is another good example of how a mismatched amplifier and speaker "will work" and even sound OK but you are not hearing the speaker sound the way the designer intended.
Looking at the impedance curve (thanks @vonhelmholtz ) the Sabrina is a very difficult speaker to drive. It is a classic example of a speaker that needs a robust amplifier to sound its best. It should be driven by an amp that can double its output with each halving of the impedance - a feat that your 300B amp cannot do. The result of this mismatch is that the speaker will sound weak in the bass. In other words, you are introducing an unintended tone control into your system. Your Modwright isn't specified into 2 ohms but it does provide good power into 4 ohms. However, it doesn't look like it was designed to power a difficult speaker like the Sabrina. BTW, you never see a pair of Wilsons at an audio show being driven by a low wattage tube amp. Most often it's a D'Agostino amp that puts out huge current at low impedances.
Apparently this setup sounds OK to you and the altered frequency response may be a better fit for your room acoustics or perhaps it better conforms with your taste in sound. If possible I would recommend that you try an amp that is rated to 2 ohms (something like a Krell, Pass, or Levinson) and see what it sounds like. Maybe you won't like it but at least you could hear your speakers sound the way the designer voiced them.