This is not a real Tjoeb 4000... After realizing that the Philips 713/723 and the Marantz CD 4000 were the same player from a mechanical point of view, and given that the Tjoeb transformation discards the internal board, keeping the power transformer and the drive mechanics only, I asked Herman van den Dungen if they could upgrade my humble Philips CD player into a world-beating Tjoeb player. They did, so I packaged the Philips player, sent it to Holland, and in a matter of days I received a new bigger parcel, containing the marvelous Tjoeb. So I'm done with tweaks, you say? Nope! Here is a list of tweaks I've tried, and I will also tell you which ones you should do as soon as possible to your Tjoeb (don't you have one? buy it, you surely won't regret it!)   Changing op-amps from OPA604 to OPA627: it's really easy, as they are socketed, so there's no need for soldering! Just be careful when handling them, as they are sensitive to ESD so a brief electro-static shock could turn these valuable op-amps into useless chips... This is really a significant upgrade, as they greatly open up the higher frequencies, adding a lot of detail without increasing sibilance. The soundstage greatly benefits, and bass seems more defined too. I'd start here when upgrading a Tjoeb, as if this improvement does not seem dramatic, you probably have some serious problems down in the audio chain, and most other tweaks would not give any benefit. Applying damping rings on Philips tubes: you can find teflon rings in any DIY shop, they fit 6922 valves just fine. The rationale behind this tweak is reducing the microphonics of tubes, even if inside a steel case they are not directly exposed to audio waves. The effect is small but noticeable: a soft of haze around instruments is reduced, so each player in the soundstage appears more defined. I don't know if this tweak is everyone's cup of tea: some listeners may appreciate a bit more fuzziness, commonly associated with tube sound. Changing tubes to Amperex Bugle Boy ECC88: wow! This is a huge step forward. The bass is incredibly improved, the default Philips tubes sound bloated and poorly defined when comparex with the Amperex's. The soundstage appears fuller, and songs get much more emotively involving. The last trace of sibilance disappears from vocals, which are silky-smooth. This is the most effective tweak that can be applied to the Tjoeb together with the OPA627 upgrade, and this combo turns the stock CD player into a completely different, better music source. Replacing power cord: I've used a Pirelli cable, not the greatest, most esoteric power cable on earth, but the effects were quite small, probably not worth the hassle. Well, the Tjoeb has a great power section, just look at all those caps on the right of the board! I must add that the Tjoeb is plugged into an industrial powerline filter that keeps most of garbage out of it, so the effects of custom power cords are probably reduced. I will investigate further this tweak with some popular DIY designs. Damping chassis: damping sheets are really useful to mitigate the flimsy Philips case. Applying pieces of automotive damping sheets on most of the internal case make it dead-sounding. Just avoid the zone above the tubes, as their heat could melt the damping compound. The CD transport is also a good zone to treat. Then I replaced the stock feet with three soft, air insulated feet (again, found in a DIY shop) and placed it between two slabs of marble, so that external vibrations are kept out of it as much as possible. To further dampen the marble slabs, they are covered with damping compound so that they have minimal resonance. All this weight and damping really comes into play when listening at high SPL, when you can actually feel the sound waves coming out of the subwoofer... So you just keep raising the volume, as the sounds always gets better, and so does the fun of listening music.