Glad to hear things worked out with your Sundara.
I prefer separates over combos. Separates generally have a higher potential/ceiling for sound quality and allow for incremental separate unit upgrades, which can save money in the long term. Combos can save money in the short term including on cables, avoid DAC and amp pairing, and have a neater footprint.
I think of things in terms of identifying the weakest link in your chain. Here, your Dragonfly Black appears to be a capable DAC, but wanting as an amp. An improved amp will likely sound more open and dynamic with wider soundstage and imaging, less congested and compressed - these are beyond volume aspects.
It depends on your budget, preference for separate vs combo, preference for solid state vs tube, valuing sound quality vs portability, whether you need battery powered capability. I’d advise against getting hung up on balanced vs singled ended at this stage.
For separates, in ascending cost take a look at the Magni 3+, Vali 2 with upgraded Electro-Harmonix 6CG7 tube, and Asgard 3.
Once upgrading to those amps, the weak link in your chain will be your DAC. In ascending cost, there’s the Modi 3, Modius, and Modi Multibit.
For combos, there’s the Asgard 3 with DAC module, and the JDS Labs Element 2. I prefer these over the iFi amps in this price range where sound quality is valued over portability. Where portability is a priority, the iFi products are a good option and I have a Nano BL myself.
Good luck. See link below. The headphones.com site has other articles that I’ve found helpful.