A 30 foot speaker cable is much better than a 30 foot interconnect...........It depends on many things. And over the years there have been a number of threads here in which the question of long speaker cables/short interconnects vs. short speaker cables/long interconnects has been discussed, with my perception being that a clear majority favors short speaker cables/long interconnects in most circumstances. In fact one of the advantages of monoblock amplifiers is often considered to be that they make it possible to have very short speaker cables.
Some of those dependencies: Preamp output impedance (the higher this is the more important it becomes to keep interconnects short, everything else being equal); speaker impedance (the lower this is the more important it becomes to keep speaker cables short, everything else being equal); balanced vs. unbalanced interconnects (balanced interconnects tend to handle long runs better); preamp compliance with balanced standard AES48 (which minimizes the importance of interconnect length); the internal grounding configurations of the preamp and amp (which affects susceptibility to ground loop issues, which is also affected by the resistance of the shield or other return conductor in interconnect cables, and resistance is proportional to length for a given cable type); and of course the characteristics of the specific make and model of the cables.
Regards,
-- Al