Grass is greener on other side of the pond
If I need another change a tip line I would build it using Scientific Anglers Textured Third Coast Spey Tips. Guideline has made good lines (single hand WF are good), OK lines and bad lines (especially DH heads) and because they are expensive I have wasted so much that I won't buy any without weighing their rear and front halves to see if the weight balance is good for Spey casting. Perhaps they have understood the problem and new lines are closer what their taper pictures are?
Better bellys and very cheap as well comes taking AF Compact Skagit more tapered end. Vision Skagit bellys have same taper and they come also Intermediate and Sink 3 but S3 ends around 600gr.
I have to agree with Esa and his differentiation. I nearly came away completely meanwhile from Guideline DH shooting heads/lines for the longer/heavier rods.
Still a great fan of GL rods and SH lines. The SH lines are mainly Rio
production after specifications of GL.
We can go even further in differentiation. The GL DH heads are manufactured by Rio
and Airflo, depending on the series. Rio
production is mostly relatively constant in specifications, sadly not Airflo.
The beloved Triple-D‘s (Original and RTG) were/are Airflo production, also the 4D.
There were /are good casting lines in the series and less good lines. The reason for the latter were unacceptable variations in length to weight ratio, mass distribution, tip weight and also in suppleness versus stiffness of the coating.
Of course no shooting head of any manufacturer will be exactly the same in objective measurements. Trained arms will also feel the differences in side by side tests.
One can adapt to a given point, but the differences in those lines were absolutely out of discussion for me. I had a lot of them in hands, up to four individual heads of different models. One can be lucky or very unlucky. And of course some will never experience the differences.
The Airflo production is the problem. It seems to be more difficult to produce PU lines constantly to given specifications. I would like to have more lines made of PU, but are no longer willing to take the risk of buying the more expensive ones.
It’s not the problem of Guideline. It’s the same with Loop lines and Hardy lines, manufactured by Airflo. I could give the details, but in short: tried them, measured them and forgot them.
I don’t want to be misunderstood as an Airflo hater. I like a lot of Airflo products, even some rods. I still like the Airflo Rage lines a lot on most of my smaller DH rods and trout Spey rods for specific approaches, but I have to buy three to choose the right one.
I also like the Airflo Skagit lines, because they are less sensitive to the problems described above. And I would like to have more lines made of PU because of many reasons and tried and tested many years now, but...
Instead of a risk in Scandi for the heavier line sizes and a long 4D, I would stay with Rio
or SA. Not your question, but instead of the 4D a great substitute and even more adaptable/more versatile would be the Nextcast SalarFinder. Great casting performance and Rio
The shorter RTG Compact lines from Guideline are also Rio
Good luck !