Help me understand how to optimize bass on the Zu Definition Mk IV

Give it a go, I certainly had success to start w RBs. Just take care on physically placing them under the Zus, they're obv not fixed.

Part of the joy of this hobby (or is that pain? LOL) is both incremental impvts, and occasional big leaps. 

For me, the Def4s have so positively responded to the system wide changes I've made, they've justified me not taking the easy option and simply upgraded to pricier, more complex spkrs.

David,

I was able to speak with Sean Casey this morning. Your Def4s definitely are early enough to have to original lower-gain sub amps. You have s/n 007/008; I have s/n 009/010. You have three options, one of which requires you spend some money.

First, you can experiment with the Low Pass Filter frequency. It goes all the way to 110 Hz, so try frequencies above 60 Hz so you are well into overlap with the main drivers' bass response. I don't normally recommend this but it might work for you, and may require experimentation with the other controls to get it to sound right. You can run the level at 9, 9.5 or 10. No harm in trying.

Second, Sean says if you had Def2 and want some of the extra kick-drum slam that the 4x10" array delivered, you will get more of that out of Def4 using rubber feet instead of the stock spikes.

Third, you can simply remove your sub amp modules, return then to Zu for updating which takes about 4 days. The cost is $600/pr. They are not just making a gain change to the amp. The revision also includes Lundahl gain transformers embedded in the module. To arrange this, just call up Zu and talk to Sean or Gerritt. This will get you all the added gain you need, and you still may want to combine it with some measure of the first two options.

Phil

Thank so much, Phil. I really appreciate your help here!

Interestingly, I did try higher crossover frequencies (along with various settings of the parametric EQ) over the weekend and it was definitely an improvement. I'd still like some additional gain for more headroom, but further steps forward. With my ears not so focussed on the inadequate bass I was hearing previously, the upgraded drivers and Radian tweeters really began to shine. 

I wondered, too, if these speakers would benefit from a little less toe in (they are currently pointed directly at the listening position, only 9 feet away), compared to the mk1.5's, which were in the exact same spots. The spray of the Radians is glorious, but perhaps a little too prominent in the mix in this configuration. I haven't yet played with this, as moving these new Zu's is more than a one person job! 

I will make contact at Zu to upgrade the bass modules. I've come too far to stop now :)  

Once I get everything sorted, I'll close the loop and let you guys know how it all turns out...

Happy listening, 
David

Definition 4 setup uses less toe-in than Def 1.5 and Def 2 in every room where I have heard both. Particularly in a relatively near-field situation.  Both the Radian supertweeter and the FRD have broader dispersion than the respective drivers in the Def 1.5/2.

You can reduce toe-in with Def 4 without tearing a hole in the middle of the soundstaging. Get some help and adjust toe-in by quarter inch increments of rotation around the inside front foot or spike as the axis of rotation until you hear the soundstage lock in. You can start with no toe-in and incrementally add some or start with the toe-in that worked for Def 1.5s and incrementally reduce it. Trying from both reference points should be revealing because you want to end up in the same place regardless where you start. It helps to use recordings from before the multi-tracks / extensive multi-mics era (pre-1964) or modern recordings known to use simple mic'ing and minimal mult-tracking, to really nail it.

Phil

Also, by the way, David: To remove your sub amps, note that the Speakon connector there is flange surrounding the connector itself. Start by removing the screws from the Speakon flange. The Speakon connector and its wiring harness stay with the Definition. After you've done that, then remove the six screws holding the amp/controls assembly to the Def cabinet. That assembly pulls away and that's what you ship back to Zu.

Phil