If the marking of the varactors is "B", which can easily be misinterpreted as "8", considering the size of the packages, the component may be a BB555-02V. Yet, this diode requires a higher control voltage of up to 28V. On the other hand, its minimum capacitance is way lower than that of my previously assumend component, the BBY58-02V (2.1 vs. 4.7pF). The BB555-02V would also better fit the frequency range of this oscillator. It's highly likely that Rigol used an Infineon component here since the other diodes and the transistors are of this origin as well.
G0HZU's hint that these capacitors are actually used to adjust the oscillators during initial testing at the manufacturer makes good sense: On my unit, for the highest frequency, all three capacitors (series, parallel and feedback divider) are hand-soldered while the mid range oscillator only has got the feed back divider cap "patched" and the low-range none at all. The higher the frequency, the more critical parasitics and component tolerances get, so voila...
It's strange that your DSA815 main board is of such a bad soldering quality, actually all of the Rigol gear that I had a "peep" into, was of very decent build quality, I never found any reason to complain about that. Their firmware is a different story, though

. Yet, that's not the point here... You may have received a lemon that slipped through the quality control, but obviously, that won't console you.