help........

Try this:

https://www.parts-express.com/harrison-labs-12-db-rca-line-level-audio-attenuator-pair--266-244

This doesn't sound right - are you talking line or phono for this problem.
If phono you have a mismatch between cartridge/phono  

CJ is notorious for having a lot of gain, the classic has 27db.  The McCormack only needs 2.25 volts for its full 500 watt per channel output.  A CD or Dac output should be 2v.  So the OP should be able to get full output from a preamp that has 1db of gain.  

The only two solutions here are imhififan’s suggestion for attenuators, or a different preamp with less gain.  

@mcreyn 
If the problem exists with his Oppo 203 then the solution is to go into the set up option on the Oppo and set a max volume. This will enable him to run his CJ at a higher volume.

There is no low gain substitute for the M8080 ( 6C4 ) tube. I use this SCHIIT SYS to attenuate the output of my CDP. It’s worth a try!
https://www.schiit.com/products/sys

+yogiboy, the SYS is all you need.

If the problem exists with his Oppo 203 then the solution is to go into the set up option on the Oppo and set a max volume. This will enable him to run his CJ at a higher volume.

@dover The wording in the manual for the Oppo UDP-203 pertaining to the "maximum volume" setting in the menu leaves me uncertain as to whether it (a) allows the entire output voltage range to be scaled down, or (b) simply limits the maximum possible output voltage. Do you know for a fact which of these possibilities applies?

If (b) applies your suggestion would of course not resolve the issue.

And if (a) applies do you have some idea of how much the maximum output voltage can be scaled down? I’m thinking that something like 0.5 volts would be necessary to satisfactorily resolve the OP’s issue, and preferably even less.

Regarding the other suggestions that have been offered, an advantage of an inline attenuator such as the one Imhififan suggested is that in contrast to the Sys it would not require an additional set of cables, and with it their potential to introduce sonic effects as well as adding cost.

As an alternative to the Harrison Labs attenuator the OP may want to consider Rothwell attenuators, either the 15 db version or the 20 db version. In addition to providing more attenuation than the Harrison Labs device they have a much higher input impedance (approximately 30K vs. a bit under 9K, both figures varying slightly depending on the input impedance of the device they are driving). In this particular case either input impedance appears likely to be suitable, but the low input impedance of the Harrison Labs device might limit its usefulness in the event of future changes to the system.

The Sys, btw, has an input impedance of only 10K, which like the Harrison Labs attenuator might make it unsuitable for use in the event of future changes to the system.

Regards,
-- Al

P.S: I neglected to mention that the approximately 30K input impedance I cited for the Rothwell attenuator applies to its 10 db version (and is based on measurements I have performed). But I suspect that the corresponding figure for the 15 db and 20 db versions is even higher.

Also, btw, the input impedance I cited for the Harrison Labs attenuator is based on measurements another member reported here a few years ago.

Regards,
-- Al

I originally purchased the SYS as I thought I needed it between the preamp and amp to be able to turn up the preamp volume control a higher amount.  I then realized that my system was OK without it, i.e. that the volume control on my preamp could be turned up high enough without it.

I then happened to purchase a pair of monoblocks, and am using the tiny SYS as an enjoyable passive mini preamp, for two different sources. So for $49 + about $5 shipping you have a multiuse little gizmo.

As an alternative to the Harrison Labs attenuator the OP may want to consider Rothwell attenuators

+1  I have a couple of different values. I use them as needed.

@almarg 
Al - the max volume function limits the output volume - which means the preamp has to be turn up to achieve the same volume at the speakers.
You can also use the variable volume on the 203 to overcome the problem - the advantage of using the max volume functionality is that it prevents some idiot from winding the preamp up full volume and then pressing play - turning the speakers inside out.
If I recall correctly you can also set the volume on the 203 and then turn off the variable volume which means you cant increase the volume without going back into set up and turning the variable function back on.
 
In my view it is suboptimal to add another device and interconnects as is being suggested when it is not necessary to fix the problem .

@dover Thanks! So you've identified two approaches that can be implemented easily and at no cost. I suppose that what the OP will need to verify is that turning down the max volume or variable volume setting far enough to allow satisfactory range on the preamp's volume control does not degrade the sound. For example, as a result of what is referred to as "bit stripping." Whether or not that occurs will depend on the specific design of the Oppo, and probably also on how much either of those settings needs to be reduced.

And if perchance that is a problem, he's been given a number of other low cost alternatives he can choose.

Best regards,
-- Al