Hi

The best channel would be the one with the biggest difference between the customer life time value (LTV) an the cost of his acquisition (CAC); provided of course that there is not big limit on its throughput (some channels may simple have limit capacity i.e. your network).

Anyway you probably would have to try to be efficient and try to target them with some mass weapon (probably their LTV is lower than for big customers). My guess would be:
1. Some content marketing: Slideshare (both presentation on your product as well as around the efficiency of workers, recruiting etc.), Linkedin especially group discussion and sharing of your staff from your blog or slideshare. Good try would be probably also Youtube channel and youtube ads
2. Mass events. Conferences around the topic, networking events – there you can present yourself and strengthen your position as the expert on the topic
3. Cold calling – use Linkedin for it and their premium version
4. Partners and affiliation / success fees . Since they have low LTV talk with partners who are already selling them staff. In Poland we have for example a company selling cafeteria system / benefit programs. They can sell you directly or through their channels. Talk with SaaS providers for the small and medium companies (SME) especially connected w. A good partners could be companies selling e-learning solutions, training companies etc.
5. Integrate. Analyze the pains of the typical HR manager at the SME and build within your solution something which solves the pain for them i.e. does some sort of statistics their need for their bosses or helps them rank the candidates. You can also build heavy knowledge base where you keep ready-made solutions for them that make their work much easier.
6. Referral program. Create a system where people get something for referring you i.e. free training for something, access to nice software for longer etc.
That would my first choices. For more I would need more data on it

Best
Asen