Which base notes can be added to extend top notes?

I tried to extend 1% Ylang-Ylang in alcohol. I made 3 batches, adding 0.5% Amber to one of them, 1% amber to the second and 2% amber to the third. The Ylang-Ylang remained very brief in all three batches.

I had previously tried extending Ylang-Ylang with Sandalwood but the Sandalwood overwhelmed the Y-Y, even at t=0. I'm not very sensitive to Amber, so it didn't mask the Y-Y. But it didn't extend it either.

So far the only thing I've found to extend a scent is to disperse it in oil instead of alcohol. Evaporation is slowed and the initial scent is reduced. I get back to about the right strength at t=0 by tripling the fragrance concentration (e.g., 3% Y-Y in Jojoba oil instead of 1% in alcohol).

Bigsley, benzoin resin is mild, and vaguely reminiscent of vanilla only much more subtle. It is often used in bases, and is often found in amber accords. While I don't think you will find anything to extend the top notes (see the other recent thread on this), I do think that a mild ambery base, less prominent on the vanilla and labdanum, will probably be worth a try.

I can't remember whether you like musk, but that could be another way to go. Most synthetic musks are very mild.

The trick in extending top notes is to find a middle or base note that is 'close enough' to the top that the top notes will disappear and only the middle or base notes will remain, but you won't fully notice because the transition was seamless. This not only calls for the right middle and base notes, but also using them in the right quantity, and the ability to at least have a good instinct of what to use to extend a given top note is, imo, one of the qualities of a good perfumer.

You could, for instance, use a touch of linden blossom absolute, orange blossom absolute and some tolu balsam to carry a sweet and thick mandarin opening deep into the heart notes. A common one that is often used is to use jasmine to extend rose, and patchouli, too. Maybe a hint of bay to add a long lasting peppery nuance, and a tiny dash of vanilla and peru balsam to soften the base and add some sweet and spice. Of course this calls for careful balance, but when done right, it really smells like one unified, deep and voluptuous rose note that lasts through the whole of the fragrance. Under the hood it's a carefully coordinated olfactory illusion, but the person sniffing the fragrance doesn't see the smoke and mirrors but only the magic, so to speak.

That being said, I find that small quantites of opoponax are generally not that recognizable in a blend, and do go some length to extending it. I'd say that's the most universal ingredient i've come across, with sandalwood being a close second, but sandalwood is deceptive in its strength. Labdanum Absolute is another.. it's MUCH milder than the oil in the top notes, and so alters the fragrance less, but add too much and it will quickly flatten the heart and take over far too soon.

I would try adding some ambergris, real or synthetic. It's known to increase longevity and perfume strength, even in very small amounts... I don't have any experience with using it in fragrances myself, but perhaps someone can correct if I'm wrong. I know that the natural perfumes I've tried with ambergris always outlast the ones without it.