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

Just saw your reply Phil. My MkIV's are new to me, bought used locally. I wasn't really looking to upgrade, but stumbled upon them and... well... you know how this hobby is. They look to be the first version (not rev B, if that makes a difference). My room dimensions are 13' wide by 22' deep, with the speakers on the short wall. They are 8 feet apart, tweeter to tweeter, with the listening position 8 to 9 feet from each tweeter. Almost a nearfield setup, which might be part of the issue in the way the bass is sounding in the sweet spot. Despite the size of the room, these positions can't really be changed, as the other half of the room is a music studio for playing and recording. 

My 300b tubes are Full Music 300b/n mesh plates. I've always loved them and they churned out mind-bending bass with my previous mk1.5s. 

However, you are right that I may need to wean myself off some of the exaggerated bass from the 1.5s. Being a bass player and a lover of some bass-heavy genres like reggae, those deep frequencies just floated my boat, even if it resulted at times in some smearing of the midrange. That said, I could barely turn the amplitude knobs past 12 o'clock without blowing my head off, while now I'm struggling to get enough bass out of the MkIV's. I do listen to a wide variety of music, so I'd like to get the balance correct!

The reason I asked about new vs. used is that in the early Def4, Sean set the internal amp's gain fairly low, such that in many rooms, including mine, the bass level had to be cranked to 8+, often 9, just to get in the realm. I pointed out to Sean that in a bass-shy room a user would not have enough sub amp gain headroom to properly load some rooms relative to the FRD efficiency. So he subsequently changed the internal gain setting on the internal amp so that the external pot yielded a far greater range of useful gain. I know all the Rev.B speakers were corrected, but some original series were too. I just don't know the serial # cut-off.

Call Sean with the serial numbers of your speakers, and he should be able to tell you if that's the case with your speakers, and he can send you a fix. If you have your sub gain cranked almost to 10, I believe that's your first order problem. In most environments, floor-to-plinth gap on Def 4 is practically immaterial. There are some situations where it is conceivable to matter but I have yet to actually encounter it. It doesn't hurt to experiment but I think you won't have to.

The TJ / Full Music Mesh Plate 300B (really a perforated plate) has three distinctive characteristics, compared to a more objective tube: a rich and euphonic top-end spray, classic coveted 300B lush midrange, and comparatively bloated bass that easily infects midrange transparency and articulation. It's a rich, creamy, decidedly not-objective sound. Now, all the Definitions have internal sub amps that derive their input signal from the bass character of the full range power amps. But the Def4 12" downfire is capable of much better bass articulation and is more dynamically disciplined, so you will have more to work with if you switch to one of the better solid plate 300B tubes, chief of which I suggest the KR 300B balloon glass. But if you need to spend less, one of the Sophia or TJ solid carbon plate tubes, or even the Shuguang Natural -T will be better.

IIRC Sean suggests new owners of Def4 start with the LPF set to 60 hz, which is right for almost nobody, but working down from there gives you an audible ramp into what works in your room. Then you can dial in amplitude and go after the PEQ settings to refine. But it seems highly-likely to me that you have an early pair of Def4 that made it into the field before Sean altered the internal amp gain to a more useful level.

Check and report back, or send my your serial #s and I'll call him tomorrow.

Phil

Phil... wow, amazing feedback! Yes, the mkIV's seem bass shy at reasonable volumes against the FRD efficiency (almost as if they were tuned brighter than my mk1.5s), but can clearly crank out deep frequencies when turned up. Just checked and the serial numbers here are 007 and 008, so I'd guess these are very early models. I'm working tomorrow and don't really know the Zu guys, so if you'd have time to call and ask Sean, I'd be very appreciative! 

I suppose I've liked the 300b mesh plate because I listen to so much music that isn't that well recorded and the rich and creamy character of these tubes hides some of that... 70s reggae made in the ghettos of Kingston just isn't audiophile grade. But I also have a strong collection of jazz, blues, R&B, and rock on vinyl as well, some of which are superb recordings. So for me it has been about finding a balance over the years of a system that's capable of being highly resolving, but also forgiving of poor recordings. It's a delicate dance. I'm certainly willing to try out some new tubes though ;)

Thanks again for all your help!
David

OK, David. I'll call Sean tomorrow and circle back. At 007/8, I am now certain this is your problem. I didn't get mine corrected because "9" gets me to the right spot, but if I moved while owning these I'd surely have to get the change myself. Standby.

Phil

Interesting and useful info from Phil. My Def4s are also a very early pair. Sean probably should update the downloadable owner's manual from the Zu website.

Gary