Upgrade from Pass Labs XA30.8 to X250.8. Worth it?

Haha, well, I just confirmed what I heard in this review of the BAT 51se: 

"The balanced output impedance was twice that specified, at 410 ohms over most of the audioband. This is low, though it did rise to a high 4.7k ohms at 20Hz, due to the size of the output coupling capacitors. (There is only so much room within the chassis for these high-performance but physically large components.) This will not be an issue with components having high input impedances, such as BAT's own power amplifiers, but it does mean prematurely rolled-off low frequencies with low load impedances (fig.1), such as those offered by some solid-state amplifiers."

Read more at https://www.stereophile.com/content/balanced-audio-technology-vk-51se-line-preamplifier-measurements

Now the question is, how do I fix it without replacing a preamp?

@bdp24 I was thinking along the same lines. I was hoping to do it without replacing the SVS. I've been searching for a high-level to RCA input converter, but haven't found a suitable solution yet. 

But you might be right. Maybe REL is where it's at. I am a little surprised that SVS doesn't offer this type of input solution, especially on their top-of-the-line unit. I have to think that more preamps would have this issue than just mine. Could be wrong though. 

Your calculation of the 18K is correct, and I feel pretty certain that your diagnosis of the bass issue is correct. The 18K load impedance divided by the 4.7K output impedance at 20 Hz is a ratio of 3.8, well under the rule of thumb guideline of a minimum of 10. And the wide variation from 410 ohms to 4.7K makes that situation particularly bad. (If the output impedance were 4.7K throughout the audio band the consequences would be mainly a slight gain reduction and increased sensitivity to cable effects, and perhaps a small degradation in the distortion performance of the preamp. While the large difference between the output impedance in the bass region and at higher frequencies, in conjunction with the heavy load impedance, will cause both frequency response issues and frequency-dependent phase shifts in the bass region).

To add to bdp24’s suggestions, a few years ago another member here, who uses Audio Research Reference series electronics, very successfully resolved a similar issue by having a tube-based buffer stage, providing suitably high input impedance and low output impedance, custom made for him by Tom Tutay of Transition Audio Design in Florida. It would be inserted between the preamp and the sub. I believe the cost was considerably less than $1K, although in his case balanced interfaces weren’t necessary. I don’t think Tom has a website, but a Google search should provide you with his contact information.

Good luck. Regards,

-- Al

A slightly old thread but thought I'd comment.  I have an XA30.8 (which itself was an update from an XA30.5) and had upgraded my speakers to Tekton Double Impact in my living room/primary setup.  The tonality was great, but I wanted to get more of that super-sharp imaging feeling/a bit more accurate reproduction, so after a few questions to Mark at Reno HiFi he strongly suggested trying the X250.8.

I did and have been quite happy with the results.  Mark may have found the perfect setup for me in the X250.8 + Tekton DI.  I haven't really noticed any degradation in tonality (but I never get anywhere close to the end of the class A envelope of the X250.8 even in my large living room at reasonable listening levels), but I *have* noticed the increase in imaging clarity, bass reproduction, and sense of crisp/dynamic control across the whole range.

I still have the XA30.8 in my secondary setup with some Amphion Argon 2's and the sense of lush tonality is great when I'm in the mood for it.

So I think it depends on what you're after.  The XA30.8 is already a bit of a move away from the "lush class A" sound in the XA30.5 I'd had before, but I enjoy accuracy of reproduction.  The X250.8 seems to be where they tried to give it yet more control and sharper reproduction while keeping at least some of the XA.8 magic.  Maybe that describes the differences between the X and XA series in previous generations as well, but at the .8 series it seems to have found a point where I'm pretty happy with the X and XA lines, with the X250.8 being my personal favorite so far.