Adding Tone Controls?

Your system description covers two different configurations, and appears not to have been updated in many years. Are you presently using biwired Goertz MI-2 speaker cables, as shown in one of the descriptions? And if so, are you using them without a Zobel network?

If so, between the ultra-high 950 pf/foot capacitance of those cables, which as seen by the amplifier would be doubled in the biwired configuration, and the very low impedances and highly capacitive phase angles which I presume your M-L Odyssey speakers have at high frequencies, you would be subjecting the amplifier to brutally capacitive load conditions at high frequencies. While normally I would be among the last to suggest that the symptoms you are describing be addressed by focusing on cables, if the answers to my questions above are "yes" it seems very conceivable to me that the effects of all that capacitance on the amplifier could be a significant contributor to the problem, exaggerating the consequences of excess high frequency energy when it is present in the recordings.

As far as digital EQ is concerned, you may also want to consider the DSPeaker Anti-Mode 2.0 Dual Core, at around $1100, which has been getting a lot of good press. (I have no experience with it). Among many other functions, as described in Kal Rubinson's review (scroll down to the middle of the page) it "includes a 16-band, user-configurable parametric equalizer with a center frequency range from 20Hz to 24kHz, each band assignable to the right, left, or both channels."

Based on reviews and comments I have seen its transparency is quite good, but if you go that route I would, as suggested by some of the others above, still configure the system so that it could readily be switched completely out of the signal path when desired.

Regards,
-- Al

Room correction electronics such as the recommended Antimode Dual Core 2.0 ... would seem to impose their changes to the system all the time, for all recordings .... An equalizer of some sort that can be switched in or out as needed seems the best recommendation I've heard so far.

See my post above, dated 11-23-12. As I understand it the DSPeaker Antimode Dual Core 2.0 can be used purely as a parametric equalizer, albeit a very sophisticated one that operates in the digital domain internally (while providing both analog and digital interfaces), and I see no reason that it couldn't be installed in your system such that it can be completely removed from the signal path, when desired, at the flick of a switch.

Regards,
-- Al

Impedence issues between the CJ preamp and Pass amp were (and maybe still are) a concern. I raised this issue a while back on another forum and received feedback from several folks (including Kal Rubinson) confirming that the pairing meets the guideline that "the input impedance should exceed the output impedance of its source by 10x or more".

Like you, I am uncertain. The 10x factor should be applied to the worst case (maximum) output impedance of the preamp at any audible frequency. For a tube preamp that will commonly be at 20 Hz, due to the output coupling capacitor most (but not all) tube preamps use. Per Stereophile's measurements the original version of the C-J 16LS had a worst case output impedance (at 20 Hz) of 1.8K, and from their comments in the addendum to the review it sounds like the upgrades that were incorporated in the Series 2 version would not have affected that. The manual for your X250 indicates that the input impedance of its balanced inputs is 22K, while not indicating an input impedance for the unbalanced inputs which I presume are provided and you are using. Conceivably the unbalanced input impedance could be half the value of the balanced input, or 11K, which would result in a SLIGHT rolloff of the deepest bass when driven by an output impedance that rises to 1.8K at 20 Hz, while having significantly lower values at higher frequencies.

Re the use of digital equalizers: I did some further checking, and it appears that the two digital outputs of your transport are both 75 ohm coaxial outputs, on an RCA connector and a BNC connector. It also appears that neither the Behringer DEQ2496 or the DSPeaker will accept coaxial digital inputs. Also, the DEQ2496 does not provide unbalanced analog inputs or outputs, so it could not be inserted into a processor/tape loop on your preamp without converting or adapting the signals from unbalanced to balanced form, and back, which would introduce additional cost and/or possible sonic compromise depending on how it is implemented.

The DSPeaker device provides unbalanced analog i/o's, as I and others indicated earlier, and could be readily introduced into one of the processor/tape loops that is provided on your preamp. The one possible issue I see with respect to that approach is that the output impedance of the preamp's processor/tape outputs is not specified, and the input impedance of the DSPeaker's analog inputs appears not to be specified. You might want to contact the two manufacturers to ask them if they can supply those numbers. Although given that the DSPeaker would only be in the signal path when you are listening to the problematic low quality recordings, a less than ideal impedance match may not matter anyway.

Finally, in case you want to consider other kinds of digital equalizers, which could accept the signal from one of the transport's coaxial outputs, I would be cautious about assuming that both outputs can be used simultaneously, unless the manual or other literature explicitly indicates that they can be. It seems conceivable to me that the RCA and BNC connectors might simply be jumpered together inside the rear panel of the transport (rather than being driven by separate output stages), the expectation being that only one of them would be used. In that situation, using both at once would result in a severe impedance mismatch, which would undoubtedly degrade sonics for both outputs.

Hope that helps. Regards,
-- Al

The transport is also equipped with and AES/EBU output, and seems to be driven along with the coax. I know since I was able to do some A/B testing between the Enkianthus and the Musical Fidelity DACs.

Good! The rear panel photo I was looking at earlier must have been misidentified, but I now see some photos showing the AES/EBU output.

So that would provide you with a means of using the Behringer unit. I can't tell for sure from the photos and writeups I've found if the transport provides an optical output as well, though. If not you would still have to use the DSPeaker in an analog processor/tape loop, since its only digital input is optical.

Swanny, thanks very much for the info about the input impedance of the DSPeaker. I've been considering giving it a try in my system as well, at some point in the next few months, to deal with a room-related suckout I have in the 40 to 50 Hz area. Still a little concerned about the effects on transparency of introducing A/D/A conversions into the main signal path, however, although the very positive comments from Kal, REG at TAS, Roscoeiii here, and others, leave me very tempted.

Regards,
-- Al