I agree with the others, that what you are describing sounds like a gain difference, not an impedance compatibility issue.
However, according to this datasheet the 2.5i version of the preamp has a specified output impedance of 200 ohms, the same value as you indicated for your 2.5, yet under the "Features" tab of this description it is stated that the 2.5i "drives amplifier loads down to 45kohm without audible bass roll-off." Which would seem to say that the output impedance rises to much higher values than 200 ohms in the deep bass region, no doubt as a result of its output being coupled via a capacitor. (The impedance of a capacitor increases as frequency decreases).
To assure that there won’t be an impedance compatibility issue, load impedance should be at least 10x greater than output impedance, at the audible frequency for which output impedance is highest (which is generally 20 Hz if the outputs are capacitively coupled). That doesn’t mean that there will necessarily be a problem if that criterion is not met, it means that there won’t be a problem if it is met. Whether or not there will be a problem depends on how the output impedance varies as a function of frequency.
If you have not sensed any deep bass rolloff with the Aragon/VTL combo, other than what might be expected to result from the speakers, I suspect that improving the impedance match in some manner is unlikely to be worthwhile. Despite the fact that the Aragon’s input impedance is much lower than 45K.
FWIW, though, another member here (Bifwynne), who uses ARC Reference series electronics and whom I know to be an astute and discerning listener, faced an impedance compatibility issue a few years ago involving the need for his preamp to drive a subwoofer having low input impedance as well as his power amp. He resolved the issue by having a gentleman named Tom Tutay, of Transition Audio Design in Florida, custom make a suitable buffer stage for him, at modest cost (hundreds of dollars, not thousands). I don’t think Tom has a website, but his contact info can be found via a Google search.
Regards,
-- Al
However, according to this datasheet the 2.5i version of the preamp has a specified output impedance of 200 ohms, the same value as you indicated for your 2.5, yet under the "Features" tab of this description it is stated that the 2.5i "drives amplifier loads down to 45kohm without audible bass roll-off." Which would seem to say that the output impedance rises to much higher values than 200 ohms in the deep bass region, no doubt as a result of its output being coupled via a capacitor. (The impedance of a capacitor increases as frequency decreases).
To assure that there won’t be an impedance compatibility issue, load impedance should be at least 10x greater than output impedance, at the audible frequency for which output impedance is highest (which is generally 20 Hz if the outputs are capacitively coupled). That doesn’t mean that there will necessarily be a problem if that criterion is not met, it means that there won’t be a problem if it is met. Whether or not there will be a problem depends on how the output impedance varies as a function of frequency.
If you have not sensed any deep bass rolloff with the Aragon/VTL combo, other than what might be expected to result from the speakers, I suspect that improving the impedance match in some manner is unlikely to be worthwhile. Despite the fact that the Aragon’s input impedance is much lower than 45K.
FWIW, though, another member here (Bifwynne), who uses ARC Reference series electronics and whom I know to be an astute and discerning listener, faced an impedance compatibility issue a few years ago involving the need for his preamp to drive a subwoofer having low input impedance as well as his power amp. He resolved the issue by having a gentleman named Tom Tutay, of Transition Audio Design in Florida, custom make a suitable buffer stage for him, at modest cost (hundreds of dollars, not thousands). I don’t think Tom has a website, but his contact info can be found via a Google search.
Regards,
-- Al